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THE INTERNAL 
COMBUSTION ENGINE 



The Internal 
Combustion Engine 

Being a Text Book on Gas, Oil and 

Petrol Engines, for the Use of 

Students and Engineers 



By 

H. E. WIMPERIS, M.A. 
n 

Whitworth Scholar, 
Formerly Scholar of Gonville and Caius College, Cambridge, 

Associate Member of The Institution of Civil Engineers, 
Associate Member of The Institution of Electrical Engineers 




NEW YORK 

D. VAN NOSTRAND COMPANY 

23 MURRAY AND 27 WARREN STREETS 
1909 






.^ 



Butler & Tanner, 

The Selwood Printing Wori:s» 

Frome, and London. 



13 



S5> 



9~/ 




PREFACE 

The Internal Combustion Engine is of such obviously 
growing importance that its study has become a necessity. 
Everywhere one finds evidence of the gradual replacement 
of steam plant, particularly in those cases where power 
users are in a position to avail themselves of the superior 
economy in moving and standing charges of the suction 
producer and gas engine. In marine propulsion the posi- 
tion of the steam engine is at present almost unassailed, 
but even there the situation is beginning to change. 

It is remarkable that there should be no English text- 
book on the subject of the Internal Combustion Engine. A 
few short chapters dealing with it have often been 
included in text-books on the steam engine, but the subject 
has now become so important as to demand individual and 
exclusive attention. The present book is an endeavour to 
fill this gap. It deals with subjects in the borderland 
between the several allied sciences (notably physics and 
chemistry) and the exclusively practical sides of their 
application. It is hoped therefore that the student will 
be helped to understand something of the applications 
of those heat engines which work on the internal 
combustion principle, and the practical engineer to a 
better realization of the scientific principles concerned 
in the design and working of gas, oil and petrol engines. 
In order to economize space, and since it has been amply 
dealt with by many other writers, little is said of the historical 
side of the subject, nor has it been possible to include any 
discussion of the theory of the gas turbine. The treat- 
ment is necessarily mathematical in certain parts, but 
it requires nothing more than average acquaintance with 
mathematics, particularly if the reader who goes through 
the book for the first time omits the portions printed 
in small type. Acquaintance with the elements of the 
calculus is now so widespread that space has not been 
taken up by the adoption of methods which would avoid 



vi PREFACE 

its use. The introduction, into the theoretical treatment 
of the subject, of the principle of the now recognised 
variability of specific heats with temperature has involved 
the breaking of much new ground, so that it is impossible 
to expect complete success in avoiding mistakes and slips 
in the mathematical calculations. I shall therefore be 
very glad to have brought to my notice any corrections 
that may be found necessary. 

Scattered throughout this volume will be found a number 
of problems for solution. They are chiefly drawn from the- 
examination papers of the Board of Education and the Royal 
College of Science (Imperial College of Science and Tech- 
nology) with both of which the author has had experi- 
ence as an External Examiner. A number have also 
been taken from the papers set for the Mechanical Sciences 
Tripos at Cambridge. 

In writing this book so many original papers and treatises 
have had to be consulted that it is not easy to make 
the requisite and proper acknowledgments. First, how- 
ever, it is a pleasure to me to acknowledge my very great 
indebtedness to Professor Perry, to whom, as a student 
many years ago, and on numberless occasions since, my 
thanks are due for guidance, counsel and help generously 
placed at my disposal. I have also to thank Mr. Dugald 
Clerk and Professor Hopkinson for very kindly sending me 
copies of their valuable papers. I am indebted also to Mr. 
H. L. Burrell, Assoc. M. Inst. C.E., for checking the mathe- 
matical calculations and for working out the examples. For 
the illustrative matter I have to thank the Institutions, 
Firms and individuals mentioned in the following list, 
but chiefly my friend Mr. F. Strickland and Messrs. Chas. 
Grifhn & Co. for permission to reproduce certain illustrations 
from their excellent treatise on " Petrol Motors and Motor 
Cars." Finally I have to tender my thanks to the Editors 
of The Engineer and Engineering for permission to repro- 
duce certain parts of articles contributed to their columns. 

H. E. W. 

Chelsea, 

IZth August, 1908. 



To whom the Author is indebted for 
Illustrations 



The Institution of Civil Engineers. 
The Institution of Naval Architects. 

The Institution of Engineers and Shipbuilders in Scotland. 
Prof. W. Watson, F.R.S. 
The Albion Motor Car Co., Ltd. 
Messrs. W. Beardmore and Co., Ltd. 
The Bosch Magneto Co., Ltd. 
The Campbell Gas Engine Co., Ltd. 
The Crosby Steam Gauge and Valve Co., Ltd. 
Messrs. Fielding and Piatt, Ltd. 
Messrs. Richard Hornsby and Sons, Ltd. 
The Lanchester Motor Co., Ltd. 
Messrs. Mather and Piatt, Ltd. 
The Mirrlees Watson Co., Ltd. 
The National Gas Engine Co., Ltd. 
Messrs. Paraffin Carburettors, Ltd. 
Messrs. Richardsons, Westgarth and Co., Ltd. 
Messrs. John I. Thornycroft and Co., Ltd. 



ERRATA 

PAGE 

vii. Add "The Editor of The Commercial Motor ". 
xi. For line 2 read : — 

f 1 yard = 91 -44 cm. 
1 1 metre = 39*37 inches, 
xiii. For J C p = specific heat at constant pressure. 
[C v = specific heat at constant pressure. 
Read J C p = specific heat at contant pressure. 
| C v = specific heat at constant volume. 
20. For S + .50 read ST.50 

32. Lines 2 and 4. For "h.t.u." and "h.h.p." read 
"b.t.u." and "b.h.p." 

64. At head of table, for —P— read — %- 
V oV 

69. In line 6, for —p.dV read ~^~p.8V 
J J 

77. In line 10, for " correction " read " convection ". 

In line 28, for " always " read " already ". 
102. In Equation (3), the 8, C and sin should all be 

on the alignment of the dots that follow the 

equation. 
216. Inline 7, for "hydroscopic" read "hygroscopic ". 
246. To title of Fig. 80, add " By the courtesy of the 

Editor of The Commercial Motor." 



CONTENTS 

PAGE 

Author's Preface ........ v 

Tables of Constants and Symbols Used xi 

CHAPTER I 
Introductory ......... 1 



SECTION I— THEORY 
CHAPTER II 



Thermodynamic Cycles 



Manner of Working of Internal Combustion Engines 
— Units — Perfect Gases — Isothermal Expansion — Adi- 
abatic Expansion — Entropy — Constant Temperature Cycle 
— Constant Pressure Cycle — Constant Volume Cycle — 
Thermal Efficiency. 

CHAPTER III 

Combustion and Explosion . . . . . .36 

Chemical Combustion — Dugald Clerk's and Grover's 
Experiments on Explosion — Discussion of Results — In- 
crease of Specific Heat — Dissociation — After-burning — 
Later Experiments. 

CHAPTER IV 

Thermodynamics ........ 62 

First Law of Thermodynamics — Second Law of Thermo- 
dynamics — Rates of Cooling — Form of Adiabatic Law 
with Variable Specific Heat — Analysis of Certain Experi- 
ments — Measurement of Cylinder Temperatures — Revision 
of the " Air Standard " — Later Measurements of Specific 
Heat — Flow of Heat through Metal Walls of Cylinder— 
Appendix. 



x CONTENTS 

SECTION II— GAS ENGINES AND GAS PRODUCERS 

CHAPTER V 

The Gas Engine ........ 115 

Types of Gas Engine — Methods of Improving Efficiency 
— Indicators, Old and New — Heat Balance Sheets — En- 
gine Tests — Governing — Cyclic Irregularity — Balancing. 

CHAPTER VI 

The Gas Producer ........ 173 

Theory — Typical Suction and Pressure Producers — 
Tests — Costs — The Gas Producer adapted to Marine Pur- 
poses — Appendix giving mode of Operation of Suction Gas 
Plant. 

CHAPTER VII 

Blast-Furnace and Coe:e-Oven Gases .... 208 
Thermal Value — Cleaning — Utilization of Surplus Power. 

SECTION III— OIL AND PETROL ENGINES 

CHAPTER Vin 

The Oil and Petrol Engine . . . . . . 223 

Fuels — Slow-speed Oil Engines — Diesel Engine — Petrol 
Engines — Carburettors — Governing — Theory of Jet Car- 
burettors — Ignition — Appendix giving mode of working 
of H. T. Magneto. 

CHAPTER IX 

Petrol Engine Efficiency and Rating . . . .291 

Efficiency Tests under various conditions — Effect of 
Cylinder Dimensions on Efficiency — Engine Rating — 
R.A.C. Rule — Callendar Rule — Composition of Exhaust 
Gases as related to Efficiency — Road and Air Resistance — 
" Gross-Ton-Miles-per-Gallon " Measurement. 

Index . . . 321 



TABLE OF CONSTANTS. 



1 inch=25-4 -millimetres or mm. 

1 yard= 39-37079 inches. 

1 litre=l cubic decimetre =1,000 cc.'s, and 1,000 cc.'s of water 

weigh one kilogram. 
1 cubic metre = 1,000 litres. 
1 cubic centimetre of water weighs one gram. 
1 gallon=01605 cu. ft. = 10 lb. water at 62° F. 
1 knot= 6,080 ft. /hour. 

Weight of 1 lb. in London= 445,000 dynes. 
1 lb. avoirdupois= 7,000 grains= 453-6 grams. 
1 cu. ft. water weighs 62-3 lb. 
1 cu. ft. air at N.T.P. — Normal temperature and pressure (0°C. and 

760 mm.)— weighs 0-0807 lb. 
1 cu. ft. hydrogen at N.T.P. weighs 0-00557 lb. 
1 ft.-lb. = l-3562xl0 7 ergs. 
1 h.p.-hour= 33,000x60 ft.-lb. 
1 electrical unit= 1,000 watt-hours. 
Watts == volts X amperes. 
1 h.p.= 33,000 ft.-lb. /min.= 746 watts. 
1 atmospheres 14-7 lb. /in. 2 = 2,1 16 lb. /ft. 2 = 760 mm. of mercury 

= 10 6 dynes /cm. 2 nearly. 

A column of water 2-3 ft. high corresponds to a pressure of 1 lb./in. 2 .. 

Absolute temperature T=273-7° + £° C. 

22 
tt= 31416 or roughly — . 

1 radian=57-3 deg. 

To convert common into Naperian Logarithms multiply by 2 -302 6,. 

in other words 2-3026 X Log. 10 a;=log = - x. 
e =2-7183. 

g =32182 ft. /sec. 2 or 981 cm. /sec. 2 at London. 
1 gallon= 4-537 litres. 
1 cu. ft. = 0-0283 cu. metre. 
1 lb./in. 2 =00703 kg. /cm. 2 . 
1 inch water=0187 cm. mercury. 
1 kg. /cm. 2 = 14-22 lb./in. 2 or nearly one atmosphere. 

1 metric H.P. = 0-986 English H.P.= 75 kg. metres /sec. 

xi /> 



Xll 



TABLE OF CONSTANTS 



1 C.H.U.= 1 lb. water raised 1° C. 

1 calorie=l kg. water raised 1° C. = 2-204 C.H.U. or 3,088 ft.-lb. 

°F.— 32 °C. 



1 B.T.U.= 1 lb. water raised l 3 F. 



9 5 

1 H.P.= 10-68 calories /min. 
1 cu. inch= 16-387 c.c.'s. 
1 cu. ft. =28 -31 litres. 
1 cu. inch cast iron =0-26 lb. 

„ wrought iron=0-28 lb. 

steel=0-28 to 0-29 lb. 

,, copper=0-32 lb. 

1 erg.= l dyneX 1 cm. 
1 gramme- centimetre = 981 ergs. 
1 ft.-lb.= l-356xl0 7 ergs. 
Energy obtainable from — 

1 lb. coal =12,000,000 ft.-lb. 

1 lb. paraffin =18,000,000 ft.-lb. 

1 lb. petrol =15,000,000 ft.-lb. 

1 cu. ft. coal gas= 550,000 ft.-lb. 
E, or Young's modulus for iron or steel=30x 10 ( 
1 cu. metre water=l metric ton=2,204 lb. 
1 H.P.-hour=641-4 calories. 



lb. /in. 



MOLECULAR WEIGHTS AND DENSITIES OF GASES. 



Gas. 






Formula. 


Molecular 
Weight. 


Density 
(Air= 1). 


Air .... 

Carbon dioxide . 
Carbon monoxide 
Ethylene . 
Methane . 
Oxygen . . . 
Water vapour 
Hydrogen 
Nitrogen 






C0 2 

CO 

C 2 H 4 

CH 4 

Oo 

Hob 

Ho 

N 2 


43-90 
27-94 
27-95 
15-97 
31-93 
17-96 
2 00 
28-02 


1-0000 
1-5197 
0-9671 
0-9674 
0-5530 
11052 
0-6218 
00692 
0-9701 



TABLE OF CHIEF SYMBOLS USED. 

;p=pressure in lb. /in. 2 . 
V= volume in cu. ft. 
T= temperature absolute (Centigrade). 

0= temperature as read by thermometer (Centigrade) in thermal 
calculations, and angle moved through in geometrical 
calculations. 
i?=heat energy in C.H.U. (Centigrade heat units). 
<f>= entropy in ranks. 

v= velocity in ft. /sec. or miles /hour as the case may be. 
Cp= specific heat at constant pressure. 
C v = specific heat at constant pressure. 
J=mechanical equivalent of heat or "Joule's equivalent." (It 
may be written as 778 ft.-lb. in Fahrenheit scale or 
1,400 ft.-lb. in Centigrade scale.) 
a=the constant term in the equation C p =a + s9. 
/5=:the constant term in the equation C v =/3 + s9. 
a t =the constant term in the equation C p =a. + sT. 
/^^the constant term in the equation C v =/3 1 +sT. 
5= the rate of change of specific heat with temperature as 

shown in above equations. 
2= time in seconds. 
77= efficiency. 

c, 

ri =gandy =| 

R=C V -C V . 

w=z weight of unit volume. 

k = conductivity for heat 

if:=zcalorific valve. 

co = angular velocity. 

1= moment of inertia. 

Si= specific heat of metal. 

n=the power in pv n equation. 

0= amplitude of temperature variation in metal skin. 

iW r =mass. 

E= intrinsic energy, 
h.p. =: horse-power. 
b.h.p. = brake horse-power. 
K.W. = Kilowatts. 



CHAPTER I 
Introductory 

1. Lecturing recently at the Royal Institution on " Flame 
in Gas and Petrol Motors," Mr. Dugald Clerk pointed out 
that although there are now in use stationary gas engines 
to the extent of over 2,000,000 h.p. and motor car engines 
to the extent of yet another 1,000,000 h.p., very little is 
known as to the actual properties of the working medium 
employed. This condition resembles that prevailing in the 
world of electricity, in which, notwithstanding the mani- 
fold uses to which the electric current is put, no one knows 
what an electric current is, or even what electricity itself is. 
At the first sight, therefore, it seems a hopeless task to write 
any " theory " of either the one science or the other ; appar- 
ently there is no foundation at all to build on, and it is as 
futile a task as the building up of the walls of a house without 
any proper solid foundation being available. Let us pursue 
this analogy a little further. Dwellers in cities are now 
familiar with the modern method of house construction, which 
starts at the top and builds downwards, or starts in the 
middle and builds both upwards and downwards. The few 
necessary steel or ferroconcrete ribs and bones are there, and 
the flesh of walling is fitted on to them. This filling in can 
be started at any point, and the house assume a habitable 
aspect after it has proceeded long enough. Thus it is with 
the study of internal combustion engines and with electrical 
machinery. In the case of the latter we start with a definite 
relationship between the volt, the ampere and the ohm. and 
from this a whole mass of useful deductions and calculations 
can be made — even although no one knows really what a volt 
or an ampere or an ohm actually is. But then it is not 

B 



2 THE INTERNAL COMBUSTION ENGINE 

necessary to know the basis of the constitution of matter — 
as exemplified, let us say, in a cricket ball — in order to be 
able to play cricket. Indeed it is a recognized fact that 
those who know most about the constitution of matter are 
not those who play cricket best. 

With the internal combustion engine we are placed in 
a parallel position : We do not know the way in which the 
properties of gases vary with temperature, and this coming 
On top of the fact that we do not know what a gas is, nor 
what temperature is, would seem to make progress somewhat 
heavy if not impossible. But as with the game of cricket, 
so it is here. We can find out some relationships between 
different properties of the substances we are using, and then 
if these relationships are sufficiently simple (as luckily they 
usually are), we can deduce from them a working theory. 
The more we calculate and the farther we get along the 
path the more necessary it is from time to time to check 
our position by direct experiment. If we get confirmatory 
results we go on, but if the facts fall outside our theory we 
must go back until we return to the last point at which 
experiment had coincided with theory, and then try again. 
This very necessary check is not always applied — and even 
when it is, experimenters not infrequently give the experi- 
ment but little chance of showing that they are wrong. 
In this way theories have often been put forward which 
have had afterwards to be abandoned. 

A theory on which practically all engineers, if over thirty 
years of age, were brought up, was that the specific heats 
of gases were constant, or sufficiently nearly so for all prac- 
tical purposes. Now, thanks to the work of MM. Mallard, 
Le Chatelier, Holborn, Austin, Hopkinson, Dugald Clerk, 
Burstall and others it has been shown that this is not so. 
It has therefore become necessary to work out a new theory 
of gas engines based on this latest information. This is one 
of the chief reasons why this book has been written. It is 
unfortunate that the fact of the specific heats of the gases 
employed increasing with temperature should make the 
calculations more difficult instead of less difficult, but the 
problem exists and it must be faced. Calculations and 



INTRODUCTORY $ 

theories should be built on true foundations, or not built at 
all. No full theory has yet been put forward. The author, 
however, makes an attempt at a beginning, and no better 
piece of work could be given to a student than to take the 
theory as the author leaves it and carry it on farther and 
farther. He must not, however, forget to check his steps 
by constant experiment, and test them as mentioned above. 
2. With these preliminary observations we will consider 
what are the sources from which is obtained the energy 
that drives our engines. They are : 

1. Solar heat (past or present). 

2. Tidal Action. 

3. Molecular action (radio-activity). 

Solar heat is available in two ways — either in the form of 
stored energy, or in the form of energy pouring in hour by 
hour and day by day. The former is best exemplified by coal 
and oil. These substances have been formed by solar heat 
acting on the earth through many thousands, tens of thou- 
sands, hundreds of thousands and even millions of years. When 
we burn oil or coal we are therefore spending capital. The 
solar radiation that is received each day by the earth is little 
used directly, although gigantic mirrors and boilers have 
been erected in some parts of the world, such as Mexico ; 
but although the heat is thus obtained for nothing, the 
apparatus required is very costly. Indirectly the Sun's 
rays are used in windmills and waterfalls, as it is simply 
the heat of the Sun acting differentially over the earth's 
surface that causes the wind, and the evaporative power 
of the Sun's rays that leads to clouds being formed and 
therefore to rain, streams and waterfalls. 

Tidal action, which is jointly due to the Moon and the Sun, 
and chiefly to the former, is little used at present owing 
to the cumbrous mechanism involved, and the chance of 
damage through storms. But it remains a possibility. 

Lastly we come to Molecular action, or Radio-activity — 
the last discovered, and the one most full of promise for the 
future. As illustrative of its marvellous potentialities the 
following extract may be given from a paper by Sir Oliver 



4 THE INTERNAL COMBUSTION ENGINE 

Lodge in the Philosophical Magazine * on " The Density of the 
Ether." Speaking of the energy locked up in the ether : — 
" This is equivalent to saying that 300,000,000,000,000,000 
kilowatt hours, or the total output of a 1,000,000 k.w. power 
station for 30,000,000 years, exists permanently, and at 
present inaccessibly, in every cubic millimetre of space." 
Also in his 1908 Royal Institution lecture, after saying 
that the mass, momentum and kinetic energy of matter 
were really those of the ether, he restated his estimate 
thus — " the intrinsic energy of 1 cu. mm. of space was 
equivalent to 1,000 tons moving with the velocity of 
light. That would mean an output of a station of 
1,000,000 h.p. working for 40,000,000 years." Any 
figures of this kind must of course be looked upon as 
very rough guesses, but it is undoubtedly the case that 
enormous stores of energy are contained even in the smallest 
particles of matter. It is difficult to get at it, and fortunate 
it is that it is so difficult, else catastrophic explosions might 
ensue from the simplest experiments. The phenomenon 
presented by radium shows however that this energy does 
sometimes leak very gradually away, and by the time that 
the leak is understood our present notions about the physical 
world may have to undergo a revolutionary change. The 
student may now ask : Why this trouble to learn about 
theories which may all prove to be fallacious ? The reply 
is that it is by studying them that he will himself be the 
better prepared to solve the problems which will arise in 
the future, and that in any case the better known theories 
of the present day have been compared with experimental 
results and conform to them very closely. It may be, of 
course, that they are sometimes little else than empirical laws 
or deductions from a brief series of experiments, but even so 
they are of use, and much has been done in certain fields 
of engineering work by such theoretical investigations : As 
witness the work of Clerk Maxwell and Hertz in electrical 
work, and Rayleigh in the theory of sound. Theoretical 
and practical work should always go hand in hand, and 

* April, 1907. 



INTRODUCTORY 5 

whenever it happens that they do not do so, difficulties arise 
from the one side or the other. 

The expansion of scientific knowledge is to be desired most 
earnestly from the point of view of an advancing civilization. 
Sir W. Besant in his Westminster remarks : — " The vanished 
civilization of Roman Britain was very far superior to any- 
thing that followed for a good deal more than 1,000 years." 
It vanished because there was not in that high degree of 
civilization sufficient knowledge of the science of machines 
to repel the barbaric invader. Our present civilization 
could only be overthrown physically by a still higher 
scientific civilization, and though we think that even that 
would be a great loss, it does not follow that the future 
ages would be of the same opinion. It is no exaggeration 
to say that prominent among the civilizing factors of our 
time is the internal combustion engine. To realize this 
we must consider the many uses to which it is put. 

3. The difference between a gas engine and a steam 
engine is that the former is an internal combustion engine, 
and the latter is not. In a steam engine combustion 
takes place in the furnace of a boiler, i.e. external to 
the working cylinder altogether. In a gas engine, or oil 
or petrol engine, however, the combustion or burning 
of the fuel takes place in the working cylinder. Hence 
the name Internal Combustion Engine. The most popular 
recent development of the internal combustion engine is 
the petrol motor car — many tens of thousands have been 
built in the last few years. Another variety at the other 
end of the scale is the gun. All cannons, guns, rifles and 
pistols are really internal combustion engines, and in part 
at least similar considerations govern their design. Another 
form of internal combustion engine which is rapidly becoming 
familiar is the large gas engine, operating on waste gases 
from iron furnaces, or coke ovens, or on gases specially pre- 
pared in gas producers. 

It is not easy to find any reliable figures as to the progress 
made in this country, but the United States Government 
has produced through the medium of its Census Bureau 
some very striking statistics as regards the use of power 



6 THE INTERNAL COMBUSTION ENGINE 

for industrial purposes in the United States. In 1870 the 
total power employed in the country was 2,346,000 h.p. 
In ten years it increased to 3,411,000 h.p. ; ten years later to 
5,955,000 h.p. By the end of the century (i.e. in ten 
further years), it was 10,410,000 h.p. And in 1906 it had 
grown to 14,465,000 h.p. Before 1890, the main sources 
of power were steam and water, but since then a great 
advance has been made in the utilization of gas power. 
Between 1890 and 1900 the use of gas power increased by 
1,400 per cent. And between 1900 and 1905 the total output 
more than doubled. 

Students will now be in a position to grasp the magnitude 
and importance of the problems which will be dealt with in 
the succeeding chapters. 



SECTION I 
THEORY 



CHAPTER II 

Thermodynamic Cycles 

Manner of Working of Internal Combustion Engines — 
Units — Perfect Gases — Isothermal Expansion — Adia- 
batic Expansion — Entropy — Constant Temperature Cycle 
— Constant Pressure Cycle — Constant Volume Cycle — ■ 
Thermal Efficiency. 

4. Manner of working of Internal Combustion Engines. 

The popularity of the internal combustion engine is now 
so marked that almost every one who is at all interested 
in engineering work is familiar with its method of working. 
The internal combustion engine, whether in the guise of 
gas engine, oil engine, or petrol engine, is to be found every- 
where ; as an instance the writer recently noticed a small 
suction plant and gas engine located at the top of a 
mountain in Germany and working quite smoothly, although 
no attendant was to be found near it. 

The working of a steam engine is well known. It 
is usually double acting and every outward stroke of the 
piston is a working stroke and every return stroke is what 
would in gas engine parlance be called a scavenging stroke. 
In an internal combustion engine this is all changed, and 
in the most familiar type air is taken into the gas engine 
cylinder (and with it some gas, oil or petrol * in order to 
heat the air by explosion and so allow work to be done 
by expansion) on the outward stroke of the piston, com- 
pressed on its return stroke, and explosion is then made 
to occur by an electric spark. The exploded mixture then 

* In America paraffin is called kerosene, and petrol is known as 
gasoline. 



10 THE INTERNAL COMBUSTION ENGINE 

expands and does work on the outward stroke, whilst on 
the final return stroke it is ejected from the cylinder. 
This process is then repeated. Four strokes therefore go to 
a working cycle (which is therefore often called a four-stroke 
cycle) instead of two as in a single acting steam engine,* 
so that for the same maximum pressure a gas engine would 
require to be twice as big as a steam engine, both being single 
acting, to give the same power ; but actually the maximum 
pressure in a gas engine is far higher than in a steam 
engine, so that the power is not so disproportionate to the 
size. Most of the older gas engines were single acting ; 
that is to say, the pressure was allowed to act on one 
side of the piston only, but there are now many double 
acting gas engines made, and, as this enables the diameter 
of the cylinder to be reduced, it means an increase in 
the power of the engine for a given weight. The four- 
stroke cycle above described is the famous Otto Cycle. 
Many years ago Mr. Dugald Clerk invented a two-stroke 
cycle in which the suction and compression were done in 
another chamber and not in the working cylinder. This 
is called the Clerk Cycle, and it enables every outward 
stroke of the piston to be a working stroke just as in the 
steam engine ; and if the gas engine also be made a double 
acting one the proportion of working strokes becomes 
exactly the same as in a steam engine, and problems of 
uniformity of torque and balancing can be solved in just 
the same way. 

5. There are therefore two possible cycles on which the 
internal combustion engine as at present devised can work, 
either on the Otto or the Clerk cycle. When one of 
these cycles has been selected to work with, there are 
three variations in the procedure which may be adopted. 
The heat liberated by the chemical combination which 
occurs on explosion may be so utilized that it is given to 
the gaseous mixture (which is always chiefly air) either 
at constant temperature, at constant pressure, or at con- 

* A single acting engine is one in which pressure is admitted to 
one side of the cylinder only. In a double acting engine it is ad- 
mitted to both sides. 



THERMODYNAMIC CYCLES 11 

stant volume. Any one of these three methods (they also 
are called cycles, thus the " constant-pressure cycle ") 
•can be used, giving six possible combinations, of which 
the Otto constant-volume cycle is the most common, whilst 
the Otto or Clerk cycle in combination with the constant- 
temperature cycle would be most efficient for a given 
temperature range. The nearest approach to the latter 
at present is probably found in the Diesel oil engine, which, 
however, still more closely follows the constant-pressure 
cycle. 

It is now necessary to examine each of these three ways 
of adding heat to the gaseous mixture, in order to see how 
the efficiency is effected. 

The remarkable fact will be proved that for each and all 
of these cycles the maximum possible thermal efficiency 
is equal to an expression which depends alone upon the 
ratio of compression employed on the compression stroke. 
Ity ratio of compression is meant the ratio of the volume of 
the cylinder when the piston is as far out as it goes, to the 
volume of the cylinder when the piston is right in, i.e. 

-f . Before proceeding to prove this, it will 

clearance volume 

be necessary to define our units. 

6. The unit of heat is the heat required to raise 1 lb. of 
water through 1° Centigrade or Fahrenheit. The former is 
called a " Centigrade heat unit " or C.h.u., and the latter a 
"Fahrenheit heat unit," or sometimes a "British Thermal 
Unit " or B.T.U. The equivalent unit in the metric 
system is the Calorie, or the heat required to raise 1 
kilogram of water through 1° C. One Calorie =2-204 C.h.u. 

The unit of energy or work is the foot-pound, being the 
work done in raising 1 lb. weight one foot high. 

Dr. Joule of Manchester was the first to measure the num- 
ber of foot-pounds of energy that were equivalent to one 
heat unit. His measurements have subsequently been re- 
vised and the following figures probably represent the correct 
results as nearly as possible. The number of foot-pounds 
of energy equivalent to one heat unit is called Joule's 
Equivalent. If the heat unit be a Centigrade one. the 



12 THE INTERNAL COMBUSTION ENGINE 

value of the equivalent is 1,400 ft. -lb., but if a Fahrenheit 
one, its value is 778 ft. -lb. Calculations are often made in 
both Centigrade and Fahrenheit heat units, so that both 
values must be remembered. Their ratio is of course f . 

Continental engineers, and some in this country and 
America, work with the metric units, and the unit of energy 
then becomes the kilogram-metre. The relative value of 
the different constants is given in the table of constants at 
the beginning of the book. 

7. We know by Boyle's Law that provided the tempera- 
ture be kept constant the volume of a mass of gas will vary 
inversely as the pressure. This may be written p. V. = con- 
stant where p is the pressure and V the volume. Also we 
know by Charles's Law that under constant pressure equal 
volumes of different gases increase equally for the same in- 
crease in the temperature. Further, that if a gas be heated 
under constant pressure, equal increments of its volume 
correspond very closely to equal intervals of temperature as 
measured by a mercury thermometer. It is found by ex- 
periment that the amount by which a gas expands when its 
temperature is changed by one degree Centigrade, pressure 
being kept constant, is about ^t^ or % \ 4 of its volume at 0°C. 
From this it follows that if the gas obeyed this law at all 
temperatures it would have contracted into half its volume 
at — 137°C. and to no volume at all at — 274°C. The 
latter point ( — 274°C.) is called the absolute zero of tem- 
perature, and it is often used as a starting point from which 
to measure temperature. In that case the temperatures 
so measured are called absolute temperatures and equal 
the ordinary temperature plus 274 when working on the 
Centigrade scale or plus 461 for the Fahrenheit. Combining 
these two laws a perfect gas can be defined as one which 
follows the law 

p.V. 

— = constant 

T 

where p is the pressure (absolute), V is the volume and T is 

the temperature (absolute) ; all of these being measured on 

any scale or system that may be found convenient, with the 

proviso that whatever system is adopted for a given calcu- 



THERMODYNAMIC CYCLES 13 

lation must of course be adhered to throughout. The 
ordinary gases, such as hydrogen, oxygen and nitrogen, do not 
follow this equation quite exactly, but they do so very nearly, 
and even a gas such as C0 2 , which being relatively easily liqui- 
fiable at low temperatures does not follow it so closely, is suffi- 
ciently near to it at the temperatures and pressures met with 
in gas engines to enable it to be used without sensible error. 
Students of the steam engine will remember that if p, V, 
and T are measured on the usual system of units, and the 
working mass of gas be taken as 1 lb., the value of the 
" constant " is (C p — C V )J and is commonly indicated by 
the letter R, so that 

pV=RT=J(C-C v )T 
where J is, as usual, Joule's equivalent, and where C p is the 
specific heat at constant pressure (i.e. the number of heat units 
required to raise 1 lb. weight of the gas through 1° Cent, when 
kept at constant pressure), and C v is the specific heat at 
constant volume (i.e. the number of heat units required to 
raise 1 lb. weight of the gas through 1° Cent, when kept at 
constant volume). 

8. It is not difficult to show that the constant R must 
be equal to (C p — C V )J. Consider a volume of gas (at 
p , V and T ) confined in a cylinder of exactly one 
square foot in cross sectional area (i.e. about 13 J inches in 
diameter), and having a weightless piston above it to keep it 
in. Let the temperature increase to T 1 and the volume to 
V x keeping the pressure constant and equal to p Q . Then 
the heat units supplied to the gas must clearly be equal to 
C p ( T 1 — T ). Part of this heat goes to heat up the gas and 
the other part to do the external work of expanding from 
volume V to volume V x against a pressure of p . Now the 
heat units used to heat up the gas are equal to the 
heat that would be required for the whole operation had 
no expansion been permitted,* in which case the heating 
would have been done at constant volume and the heat 
units required equal to C V (T 1 — T Q ). 

It follows therefore that the difference between C p (T t — T ) 
and C V {T 1 — T ) must be equal to the external work done, 

* See p. 1G. 



14 THE INTEKNAL COMBUSTION ENGINE 

measured in heat units, i.e. to p (Vi — F )r«/., since the 
load on the piston is p (that is a pressure of p lb. per 
sq. foot), acting on an area of 1 sq. ft. and the vertical 
motion of the piston in feet is ( V x — V ). 

Therefore C p (T-T )-C v (T-T ) =-&-( V t —V Q ) 
or {C -C V )(T-T ) =J^{ V x - F ) 

po~Ey =J{C -^ ] (1) 



But 



Po V o PqVi 

T ~~ T 

F F F 

or — ^ -i- V =T —^ 

Substitute in (1) and 

F 

1 1 ^o 

But ^i = i? 

therefore B=J(C—C V ). 

9. The equation 



^=B=J(C—C t 



pV 
T 

is true for all values of p, F and T. If equal weights of two 
different gases be taken and in both the p and T are adjusted 
to be the same, the volumes will be in the inverse ratio of their 
respective densities. Thus 1 lb. of hydrogen will occupy a 
far larger space than 1 lb. of oxygen, but it follows from the 
above that an equality between the two gases must exist as 
regards the following expression. 

{C p — C v ) x density 
and the following table illustrates this in practice — 



THERMODYNAMIC CYCLES 



15 



Gas. 


Cv. 


Cv.* 


Density rela- 
tive to Air. 


(Cp— Cv) x 
density. 


Ho . . 

N 2 . . 
2 . . 
CO., . . 


3-4090 
0-2438 
02175 
0-217 


2-4060 
0173 
0155 
0171 


00692 
0-970 
1105 
1-520 


00694 
00687 
00691 
0-0700 



The fact that there are any differences at all is because 
these gases are not absolutely " perfect gases." The 
assumption is implied moreover that the specific heat is 
absolutely independent of temperature, and although for 
many calculations this is sufficiently nearly true, there are 
others, as will appear in a subsequent chapter, in which this 
is by no means the case. 

In problems connected with the internal combustion 
engine, one is continually coming across the symbols C p and 
C v and it is necessary at the earliest stage to get thoroughly 
familiar with their use. 

10. Another form in which they constantly recur is as the 
ratio C p -f- C v , and this ratio is known by the Greek letter y, 
(gamma), so that 

C, 



and since 





V 


=7 


<v 


-c. 


=B 


Op 


1 ■ 


R 


c v 




R 


7 


— 1 


"~cl 



The expression (y — 1) is constantly recurring. The value 
of y is usually from 1*3 to 14, the latter being the value 
for air. 

It has been said that 

pV_ 



=R 



* The student of physics will remember that Dulong and Petit 
showed that the product of C v by molecular weight is practically 
constant for all erases. 



16 THE INTERNAL COMBUSTION ENGINE 

and that R is a constant for the gas concerned. If the tem- 
perature be kept constant 

p V =RT ^constant 

and this is sometimes called the hyperbolic law of expansion. 
In reality there is no such law, and the statement is only a 
variation or simplification of the perfect gas formula. If 
however the gas really does expand according to the equation 

p V= constant 

dt will be doing external work and inasmuch as the energy 
it contains will not alter, seeing that the temperature does 
not change, heat must be being supplied to the gas at a rate 
equal to that at which the external work is being done. Sup- 
pose, however, that the conditions had been such that the gas 
did not receive or lose heat, and yet was compelled to expand 
or contract by reason of the piston being forcibly raised 
by some means or other ; how would the pressure and tem- 
perature be affected ? 

Now it was discovered by Dr. Joule that in all gas opera- 
tions the heat H supplied from outside must be balanced 
by the gain in temperature of the gas plus the external 
work done, so that 

H=C v .ST+p.SV ■ 

wmere ST and SV represent the changes of temperature 
and volume which occur in any transformation. It is most 
convenient to regard the process as split up into little 
steps and therefore to regard ST and JF as small incre- 
ments of temperature and volume. 

If we consider a case in which the gas is being neither 
allowed to gain nor to lose heat H must be zero 

or C v .$T+p.SV=0 

and from this it is easy to show by means of the integral 
calculus that pV y = constant. This transformation is called 
an Adiabatic one because no heat is allowed to pass into or 
away from the gas. 

For those who are acquainted with the calculus it may be added 
that the steps of the integration are 



THERMODYNAMIC CYCLES 17 





C v .8T + p.8V. = Q 


therefore 


Ty— — T=r~ in the limit, 


but 


pV=RT=(C p —C v )T. 


or 


%iC„-C,= P+ V.%. 


Substitute and 





or —yp= v -dv- 

dp dV 

= — 7'~y~ an d integrate. 

therefore pV y = constant, 

and this is one of the most important equations that there are in 

gas engine work. 

11. Two fundamental gas formulae are — 

pV ^constant is an Isothermal Expansion. 
pV y ^constant is an Adiabatic Expansion. 

The equation ^F y =constant only gives the value of p and 

V for this transformation, but it is quite easy to get the 

pV 
value also of T, as the relation —=R holds good for all 

transformations of whatever sort or condition. 
Thus, since p = ' 

RT 

we have p V y = — V y ^constant ; 

or T V y ~ 1 =constant, and this gives the relation between 
T and V. Again 

p 

— ) = const. 



»^f)'- 



rpy 

or ^constant, 

py- 1 

and this is the relation between T and p. 



18 THE INTERNAL COMBUSTION ENGINE 

If at the beginning of any transformation p=p Q , V=V C 
and T=T 

Then p F v =a constant, 

T V y ~ 1 =a. constant, 

Jl y 

—a constant. 



and therefore the pressure, volume and temperature at a 
succeeding state being called p v V v and T x it follows that 

v x vy= v jy 

and T V y ~ 1 =T V 7_1 

■*■ 1 ' 1 M r 

T 7 7 T 7 7 

and * ° 



P 1 7 ~ 1 Po 7_1 

12. As an example, if a gas is compressed adiabatically 
in the ratio of 10 to 1, i.e. so that it only occupies ^ part of 
its former volume, then the temperature will alter thus — 

rp y y — l—rp y y—\ 
■* l r i J o v o 

where — L =A 

7 10 

K 

so that _L = (^o) =(10)t~ 1 

and as 7 = 1-4 (say) 

J=(10)°' 4 = 2oL 

^ 

i.e. the temperature absolute increases by 151 per cent., so 

that had T been 290 (i.e. a temperature of 16° C. to start 

with, about the temperature of a room in the summer time) 

then 

T 

- 1 =2-51 

290 

.-. T x =729° abs. 
.*. resulting temperature =729 — 274=455° C. 

This explains the heating of the air which occurs when air 
is suddenly compressed as in pumping up a bicycle tyre. 
It is necessary to say " suddenly," as if done slowly the 
heat would have time to escape, and the change would be 
far from adiabatic, and would in the limit become isothermal. 



i 

THERMODYNAMIC CYCLES 19 

13. The entropy diagram is familiar, if only in appearance, 
to all who have studied the Steam Engine and will therefore 
probably be already well known to many readers. The ab- 
stract definition of what entropy is makes a rather long and 
difficult study, but it must be remembered that close scien- 
tific definitions of even the commonest things tend to become 
abstruse and to suggest a strangeness which does not adhere 
to one's familiar conception of them. So it is with entropy — 
it is most easily described by a reference to its properties 
and uses. The two ideal types of the expansion of gases 
are the isothermal and the adiabatic In the former the 
temperature remains a constant. In the adiabatic expan- 
sion the temperature varies, but what does remain constant 
is the entropy of the gas. That is to say the amount of 
entropy remains the same throughout adiabatic trans- 
formations. This is probably the simplest way there is of 
defining entropy. The measure of the entropy depends 
upon the point from which it is reckoned, just as a tem- 
perature reading varies according to whether it is measured 
from the absolute zero or from 0° C. In the case of entropy 
it is found most convenient to measure from the state at 
0° C. The amount of entropy in a substance is calculated 
thus : 

Given a mass of gas and given that a certain quantity of heat is 
put into it, how does the entropy change ? This is, in its simplest 
form, the problem which has now to be faced. 

If the certain quantity of heat is called 8H and the absolute 
temperature T, then the gain in entropy, which is usually called 50 

• dH 

is -^-, so that 

P T 

This is the mathematical definition of what is here called entropy. 

It may be, however, that the temperature T will vary during 
this change and if so the whole operation must be considered as 
divided into little parts each with its own temperature and the 
whole afterwards added up. The usual mathematical way of stating 
this is 

or in the limit and using the notation of the calculus 

dH 



r [dH 

j d*~ j T 



20 THE INTERNAL COMBUSTION ENGINE 

To evaluate the entropy for any given change in the state of the 
gas it is necessary to know how H depends on T. If in a simple 
case H varied directly with T, say H = aT (water for example) 
then the equation would be — 

I d-p = I -Jp= \ a '~T =a \ 1 7F 

or 4> = a log e T+ constant, 

and since by definition = when T = 274 

it follows that the expression for the entropy is — 
* = alog«| i 

and in this way its value could be found for any given temperature. 

If a given volume of gas has its pressure, volume and tem- 
perature changed in any way so that after undergoing several 
such operations it returns to the same state, then the values 
of its entropy and temperature will, when plotted on a sheet 
of paper, form a closed curve which has the useful property 
that its area measures the heat supplied just as the area of the 
P.V. or indicator diagram measures the mechanical work 
done. If measures are made in both cases in ft. -lb. both 
diagrams in any particular case would have the same area. 
That the T.<p diagram really does measure heat supplied by 
its area is evident, since by definition 

r T 

or T.S(p=SH 

so that H=ZT.S(p 

and 2-f-.<^> is of course the area of the diagram. In Fig. 1 
is shown a very simple entropy diagram. The gas starts at 
the point 1 ; the temperature is then increased to the point 
2, whilst the entropy remains constant — an adiabatic com- 
pression ; then the gas has its temperature kept constant 
from 2 to 3, whilst the gas receives heat and the entropy 
increases from 2 to 3 ; then from 3 to 4 the gas expands 
adiabatically as the entropy is constant and the tempera- 
ture falls to 4 ; then from 4 to 1 the temperature remains 
steady, whilst the gas gives up its heat and the entropy 
diminishes from 4 to 1, so bringing the gas back to its 
original state, and ready to go through the cycle again. 
This is the well-known Carnot Cycle, which is so often shown 



THERMODYNAMIC CYCLES 



21 



on the P. V. diagram but is so much more easily understood 
on the T.cf> diagram. What is the area of the diagram in 
Fig. 1 ? It is plain from what has been said that the 




" Fig... - 

area 2, 3, 6, 5 represents the heat taken in by the engine, 

and the smaller area 1, 4, 6, 5 that rejected. 

„ . heat utilized area 1, 2, 3, 4 

So that the efficiency or-— ,. - = _ , - 

heat supplied area 2, 6, b, 5 

2 1 
and this ratio is obviously equal to ^— 

o 



i.e. to 



max. temp, of cycle — min. temp, of do. 



max. temperature of cycle 
which is the customary expression for the efficiency of the 
Carnot Cycle. This is an instance of how simple the use 
of the T.(p diagram makes such calculations. 

14. In this last named figure all the lines were parallel to 
one or other of the axes. This was because an ideal cycle 
of the simplest nature was being followed. In Fig. 2, the 
sloping lines AB and BC have been drawn at random. 
What changes of state would they represent ? 

The line AB shows an increase of both entropy and tem- 
perature, both of them increasing at about an equal rate. 



22 THE INTERNAL COMBUSTION ENGINE 



So that heat is being given to the gas, and the temperature 
is increasing meanwhile. This is generally similar to what 
goes on during explosion in a gas engine cylinder as the 
gas takes in heat from the effect of chemical combination, 
and the temperature rises while it does so. Having arrived 
at the point B the gas now follows the line BC during which 
the gas continues to take in heat, and the temperature 
decreases. This is what would occur, on a lesser scale, in a 




Fig. 2 







gas engine cylinder were the combustion of the gas to con- 
tinue right through the working stroke instead of ending at 
the point of highest temperature, as it is now generally 
believed to do. Then to get the gas back to its original 
state the line CA is followed, and during it the gas gives 
out its heat at a nearly steady temperature, i.e. almost 
an isothermal compression. No gas engine works exactly 
on this cycle, which was one drawn at random to show how 
any cycle whatsoever can be very easily and readily studied 
by the use of the T.<p diagram. It is obvious from the 
diagram that the efficiency of this triangular cycle would be 
a low one as the area is small having regard to the tempera- 
ture variation represented. 



THERMODYNAMIC CYCLES 



23 



Gas engine indicator diagrams are often turned into 
T.(p diagrams, but it is necessary that certain precautions 



3000° 



r = 



5 
100 



Po = O 9 ATS. 



9 

7 - 



PER LB 



800 B. TH. 

0190 

I 39 FOR EXPLOSIVE MIXTURE 

i-37 FOR PRODUCTS OF 

COMBUSTION 




ABSOLUTE THERMAL EFFICIENCY =0-434- 
RELATIVE EFFICIENCY ■= 059 




should be taken in doing so. The difficulty lies in the fact 
that the working fluid does not remain in the cylinder for a 
number of cycles, but is periodically discharged to exhaust. 



24 THE INTERNAL COMBUSTION ENGINE 

and a fresh charge brought in. The cycle can, however, 
be treated as a continuous one if the exhaust gases are 
considered to have their relatively high temperature and 
pressure reduced to those of the incoming charge, the volume 
being kept constant. In an appendix to an Institution of 
Civil Engineers report*, Captain Sankey has shown a number of 
P. V and T.<p diagrams for the same gas engine cycles, and by 
the kind permission of the Council of the Institution, one of 
them is reproduced in Fig . 3 . The fine lines show the T. <p and 
P. V diagrams for an ideal engine, whilst in the shaded por- 
tion is given the same diagrams for a probable actual engine. 
The wavy part of the shaded curve shows the expansion 
period of the cycle. It has been drawn to show the cooling 
of the gas to the walls and piston during the beginning of 
expansion, and the subsequent flow of heat in the reverse 
direction during the latter part of the stroke, this effect 
dying away again at the very end of the stroke, possibly on 
account of the slow motion of the piston at that point, which 
w T ould allow the walls a greater amount of time in which to 
part with their heat. 

Before dealing with the efficiencies of the various cycles 
of working it is necessary to say something about the work- 
ing medium. The gaseous mixture that enters a gas engine 
(for oil or petrol engines the same considerations apply) 
is usually -^d ^ t an( ^ ^ ne res ^ § as > an d even when the 
proportion of air is not quite so high as this, by far the 
greater part of the mixture is simply air. Air is in fact the 
working substance, and gases, oils and petrols are used 
merely to heat it up to the point required to carry out the 
predetermined cycle of operations. So that although the 
writer gives the thermal constants, not only for air but also for 
the other gases, etc, concerned, it must be remembered that 
air is the most important factor, and that inasmuch as air 
is f nitrogen, it is the latter gas which is most concerned, 
however passively, in the working of internal combustion 
engines. The following table shows the other gases chiefly 
concerned in such operations when using various working 
substances. 

*/. C.E.Proc. Vol. 162. 



THERMODYNAMIC CYCLES 



25 





Town Gas. 


Producer Gas. 


Blast Furnace 
Gas. 


Coke-Oven 
Gas. 


CO ... . 
co 2 . . . . 
H . . . . . 
N. . . . . 

Hydrocarbons . 
B.T.U. per cub. 
ft. about . 


per cent. 

7 

2 
46 

3 
42 

600 


per cent. 

20 

9 

21 

48 

2 

150 


per cent. 
25 

6 

2 
66 

1 

90 


per cent. 

8 

2 
53 

5 
32 

540 



15. Ideal Standard Cycles. — Every one who is acquainted 
with steam engines knows that the standards of comparison 
are the Carnot Cycle or the Rankine Cycle, that is to say, 
these two ideal cycles of operation are the standards by 
which actual engines are best judged. It would be unfair to 
complain of a steam engine that gave a thermal efficiency 
of 0-25 when that ideally possible for the temperatures em- 
ployed was only 30, indeed such an engine must be greatly 
superior to any yet constructed, and although 25 per cent, 
efficiency does, it is true, mean that 75 per cent, of the 
energy is wasted, yet in reality the engine is a very good one 

•25 
as it yields — , i.e. 83 per cent, of what is ideally possible. 

'o\J 

It is this figure of 83 per cent, which should really be looked 
to. The figure of 0*25 gives little information indeed, but 
the figure of 83 per cent, shows at once that unless the 
manner of working be altogether changed there is only 
17 per cent, left to improve upon. In a steam engine the 
endeavour is to keep the cylinder hot and so prevent the 
condensation which causes the efficiency to fall below its 
possible level. In a gas engine, on the contrary, the endeavour 
is to cool by the cylinder to keep the engine from jambing 
and otherwise working badly. Clearly there is here a marked 
difference in operation, and correspondingly it becomes 
necessary to devise new standards of comparison suitable 
to the working of gas engines. 
There are Three Ideal Standard Cycles, viz. — 

1. The constant temperature type. 

2. The constant pressure type. 

3. The constant volume type. 



26 THE INTERNAL COMBUSTION ENGINE 

Each of these has been investigated by a Committee 
appointed by the Institution of Civil Engineers, and as it is 
desirable to avoid a multiplicity of methods of dealing 
with the same thing, the author will follow generally the 
procedure they recommend. * 

16. The Constant Temperature Type. — In an engine of 
this type, all the heat is taken in at the highest temperature 
and all is afterwards rejected at the lowest temperature. 
Those who have followed what has been written in this 
chapter will recognize this as the Carnot Cycle, and it can 
be proved that for the same temperature limits no possible 
treatment of a heat engine can give a higher efficiency than 





,1 


1 










■ 




W25 
Id 

a. 












































X 
% 20 

o 




1 




































1- 
< 

15 

Z 
























\ 




















tt 10 

o 

CO 




\ 






















\\ 


s p ' 


















Id 

E * 




\ 




sT>2 


















To 






^"^«» 


■=:-- 


































Pa 










.1 










i 








1 






u 

-J2500 
< 

O 


















K 2000 

X 

< 


















£■500 
< 


















a. 

s 

ujIOOO 


"D 


jv»t! 














> 


500 


1 






V 










\o 



























VOLUME 



EhTROPY 



Fig. 4. 



is theoretically obtainable in this way. The diagrams in Fig. 



T T 

4 show at once that the efficiency is — i— — - 



where T* is 



the highest temperature and T the lowest, both of course 
being reckoned from the absolute zero of temperature. T 
is always used in this book to mean temperature absolute, 
and to mean temperature as read on a thermometer. The 
diagram above referred to is of course an entropy diagram, 



THERMODYNAMIC CYCLES 



27 



commonly referred to as a " 0,(p "diagram, but as 0is being 
kept for temperatures which are not absolute it] would be 
better to say " T ,</>." A " P. V " diagram is also shown and 
any one at all acquainted with the working of steam or gas 
engines will at once recognize 
that for any given h.p. the 
cylinder would require to be 
exceedingly large and costly, 
so that the extra economy 
in the matter of coal, brought 
about by its high efficiency 
would be more than counter- 
balanced by the inconvenience 
of the size of the engine and 
by the extra annual outlay 
necessary to provide for in- 
terest and depreciation on the 
enhanced capital cost. 



< 

Ll 

co 1000 

< 

CL 

£ To 

"J 500 









T* 




























































)ls 























































•I 2 

ENTROPY 



HIO 



P? 







V 


1 


\ 




























• 






\ 


V 




\ 


^^ 
































s 


^ 






























































p.. 




p!. 1 













10 
VOLUME. 



Fig. 5. 



No gas engine works on this cycle or indeed on any- 
thing very like it. It is not therefore quoted nearly so 
often in gas engine work as in steam engine practice. 

17. The Constant Pressure Type. — In this type of 
engine all the heat is received at the highest pressure 
and rejected at the lowest pressure. 

Entropy and P.V. diagrams are shown for this cycle 
in Fig. 5, and it is easily seen from them what the cycle of 
operations really is. The heat received is clearly (T 2 — TJ 
x C pi where as usual C p means the specific heat at con- 
stant pressure, and that rejected (T 3 — T ) x C p) so that 
the efficiency equals 



28 THE INTERNAL COMBUSTION ENGINE 

(T-TJC-IT-T JC,, _ T-T,-T, +T, : 
(T-T t )C v T-T l 

Xowwhen the engine is neither taking in heat nor rejecting 
it. it must be working adiabatically. i.e. pF 7 =constant. 

Therefore Vx V* =p TV and ?£l = ?? h 

J- r 



i ■"■ 



or 



To PoK Po'W W 

sinhlarlv — = ( — ] ? 

X X T A-T T T 

Then efficiency = n = — — — - = 1 — — - 

T—T x T 2 —T t 

T 

z°T T 

m rr rri 2 

and ^ = ^ so that „=l-^_^ 

-* ^ 3 ± 2 ± 1 

T 
or *=1— T -- 



T 



T—T 1 



i 



Xow V is the yolurne ai-the beginning of compression and V { 
at the end of compression, therefore the compression ratio 

r= L and IW^\> 

Therefore , / = l— (J_) V=1--(A.Y ' 

and this giyes the yalue of the efficiency of this cycle in terms 
of /', the compression ratio. It is an interesting and impor- 
tant fact that this efficiency is independent of the tempera- 
tures and pressures attained, and depends only on the ratio 
of compression, i.e. on the relatiye sizes of the yolumes before 
and after compression. It shows too that for high effi- 



THERMODYNAMIC CYCLES 



29 



ciencies the compression must be high. It must be noted 
that T ± and T do not mean the same thing in this cycle 
as they did in the Carnot Cycle already referred to. The 
careful reader will have noted this already. The Brayton 
and Diesel engines approach most nearly to this cycle. 

18. The Constant Volume Type.— In this type all the heat 
is received at constant volume and rejected also at constant 
volume. These two volumes are the volume at ignition 
and the volume at exhaust. This cycle may also be called 
the Otto or Beau de Rochas Cycle, and it is the one on which 
practically all modern gas engines work or attempt to work. 
The diagrams in Fig. 6 show the working of the cycle. 

The efficiency is calculated in the same manner as the 















































1>Z 














































































P/ 















































































Ho 




VOLUME 
To - 



ELNTROPY 



Fig. 6. 



previous one ; heat taken m=(T 2 — T t )C v and heat rejected 
={T 3 — T )C V , where as usual C v means the specific heat at 
constant volume. 

Efficiency = n JT.—T^—jT^—T^ 



■ ■ 1 






=1 






30 THE INTERNAL COMBUSTION ENGINE 



Then as before 












T ~ 


T 
T 

X 3 


-(^r- 


r ' 


Therefore 






-Mfr 


-l 



And this it will be noted is exactly the same expression 
as before. Indeed, the Carnot Cycle can also have its effi- 
ciency expressed in exactly the same way, but it must be 
remembered that although the efficiency of all three cycles 
depends upon the degree of compression and would be the 
same in all were the compression ratios the same, yet the 
temperature ranges would be very different, and it would 
be found that the Carnot Cycle gave the least range 
of temperature for any given efficiency. The discovery 
that for the same compression ratios the same efficiency 
holds good for each of these three cycles is attributed to 
Professors Unwin and Callendar. 

In view of the simplicity of this result it is not difficult to 
understand that the Committee of the Institution of Civil 
Engineers, appointed to enquire into the matter, should have 
selected for use as the best expression for the ideal efficiency 
the form — 

In*- 1 



This expression therefore holds the place in gas engine work 
which in the steam engine is filled by the well-known 

T, 

19. The remaining point to be considered is the value to 
give to y. The gaseous mixture which works in gas engines 
depends upon whether lighting gas, producer gas, blast 
furnace gas or coke-oven gas is being employed, and with 
oil engines yet different mixtures occur. It is evidently 
impossible therefore to get a value for y which will accur- 
ately suit all engines. It must be remembered, however, 
that the working fluid always consists chiefly of air, and it 
has been considered by one school of thought that, having 



THERMODYNAMIC CYCLES 



31 



regard to the preponderance of that familiar mixture of 
oxygen and nitrogen in all internal combustion engines, 
little error could arise if it were all assumed to be air. The 
"Air Standard" for efficiency resulted. It assumes that air 
is the working fluid (and that the relatively small quantity 
of gas is merely used to heat this air by combustion), and 
that 7 has the air value of 140, so that 



1 



(t) 



This expression gives for different values of r the following 
theoretical efficiencies — 



r 


n 


2 


0242 


3 


0356 


4 


0426 


5 


0475 


7 


0541 


10 


0602 


20 


0-698 


100 


0-841 



In practice 50 to 60 per cent, of these efficiencies are 
usually obtained, and it is clear that a comparison between 
different engines can be made by noting what percentage of 
the ideal efficiency is obtained, in each case, for the com- 
pression ratio at which each works. A natural result of this 
rise of efficiency with compression is that for many years 
past there has been a movement among engine designers in 
favour of higher compression pressures. It is this move- 
ment which is the chief cause of the great advances that have 
been made in the heat economy of gas engines. Thus in 1 880 
a compression pressure of 30 or 40 lb. per sq. inch was 
usual. Now the compression pressure sometimes goes up to 
170 lb. per sq. inch when working with producer gas and 
with the Diesel oil engine as high as 500 lb. per sq. inch. 
The effect of high compression pressures is illustrated in 
practice by the following figures. According to a recent 



32 THE INTERNAL COMBUSTION ENGINE 

statement * an engine working with a compression pressure 
of 120 lb. used 11,500 h.t.u. per h.h.p.-hour, whereas one 
working with a corresponding pressure of 170 lb. used only 
9,500 h.t.u. 

20. The Council of the Institution of Civil Engineers have 
very kindly allowed the reproduction of the diagrams in 
Figs. 4, 5 and 6 from the Final Report f of the Committee 
on the Efficiency of Internal-Combustion Engines. They 
were also good enough to permit of the curve in Fig. 7 
being reproduced. It shows the heat contents for 1 lb. of air, 




30 32. 40 
5 



SO t ? 60 70 t SO 90 100 IIO 

TEMPERATURE OF AIR 



120 
50 



Fig. 7. — Curves showing heat contained in 1 lb. of saturated air at various 
temperatures. Thus tS represents the heat content of 1 lb. of dry 
air and the associated quantity of water vapour, which occupies the 
same volume as the 1 lb. of air at temperature t. 



and the associated quantity of water vapour. It therefore 
enables the observer to read off at once the heat contained 



* Mr. A. E. Porte in Proc. I.E.E., 1907. 
t I.C.E. Proc, Vols. 162 and 163. 



THERMODYNAMIC CYCLES 33 

in any weight of air at different temperatures. The ability 
to do this rapidly is very useful when a heat balance sheet 
is being made out for a gas engine run, and reference will be 
made to it later. 

EXAMPLES. 

1. State the laws of perfect gases. Explain what is 
meant by (1) absolute temperature, (2) a perfect gas, and 

P V 
prove that in a perfect gas - " -is constant. A quantity of 

gas occupying 6J cubic ft. at temperature 60° F. is com- 
pressed isothermally to -J- of its volume. It is then cooled 
at constant pressure. Find the volume of the gas when 
the temperature has been lowered in this way to 32° F. 
Arts. 2-05 cu. ft. (Cambridge B.A., 1904.) 

2. A vessel is exhausted of air to a pressure of 12 lb. abso- 
lute, the pressure of the atmosphere being 15 lb. absolute. 
The temperature of the whole being that of the atmosphere 
(60° F.), a cock is opened and air allowed to rush in until the 
pressure is equalized. Assuming that no heat is lost to the 
walls of the vessel, find the rise of temperature of the air 
within it. Ans. 130° F. (Mech. Sc. Tripos, Parti, 1905.) 

3. Air expands under a piston from a volume of 1 cubic 
foot and pressure 300 lb. per sq. inch absolute to volume 
5 cubic ft. and pressure 40 lb. per sq. inch absolute. Assum- 
ing that the pressure and volume vary during the expansion 
according to the law PV n = const., find the heat absorbed in 
the process in British Thermal Units. Mechanical equivalent 
of heat =778 foot-pounds : ratio of specific heats of air = 
1-41. Ans. 74 B.Th.U. (Mech. Sc. Tripos, Part I, 1904.) 

4. The entropy of 1 lb. of water at 0° C. is 

. 273+0 

log. — . 

&t 273 

What is this if is 160° ? If this water is converted into dry 
saturated steam at 160° C, what is the additional entropy \ 
Ans. 0461 and 114. (B. of E., 1907.) 

5. Sketch the compression, ignition, and expansion parts 

D 



34 THE INTERNAL COMBUSTION ENGINE 

of a gas engine diagram. If the volumes and pressures at 
four points on the diagram, to any scales whatsoever, are 
represented by — 



Points . 


A 


B 


C 


D 


Volumes . 


6 


1-7 


2 


4-5 


Pressures . 


1 


5 


13-8 


32 



and if at the point A we know that the temperature is 140° C> 
what are the temperatures at the other points ? Tabulate 
your results. Ans. 140° C; 313° C; 1,630° C; and 720° C. 

6. In a gas engine cylinder where v=2-2 and p = l4t!2 it 
was known that the temperature was 130° C. What is the 
temperature when p=l22 and v = l'2 ? Ans. 1,552° C. 

7. What is the law connecting the pressure, volume and 
absolute temperature of 1 lb. of air ? Consult the printed 
table furnished you, for the density of air. Why is the 
specific heat greater at constant pressure than at constant 
volume ? (B. of E., 1900.) 

8. What is the law connecting the pressure, volume and 
absolute temperature of 1 lb. of air ? 1 lb. of air at two 
atmospheres pressure and £0°C, what is its volume?. 
It receives heat energy equivalent to 1,000 foot-pounds,, 
its volume remaining constant ; find its new pressure and 
temperature. The specific heat of air at constant pressure- 
is 0238. Ans. 66'75cu. ft., 115 atms., and 26° C. 

(B. of E., 1900.) 

9. A gas engine works on an ideal cycle with adiabatic- 
compression and expansion, receiving and rejecting heat 
only at constant volume. Obtain the expression of its effi- 
ciency. In such an engine the piston displacement per stroke 
is 1 cubic foot, the clearance volume 2 cubic foot, and at 
the beginning of compression the temperature of the cylinder 
contents is 600 F. abs., pressure being atmospheric. The 
engine receives 6 cubic foot of gas per cycle (calorific value 
600 B.Th.U. per cubic foot). Atmospheric pressure = 147 lb.. 
per sq. in. 



THERMODYNAMIC CYCLES 35 

Find : — (a) Weight of cylinder contents. 

(b) Pressure and temperature at end of compres- 

sion (take 7=1*38). 

(c) Rise of temperature during explosion (neglect 

jacket loss and take C v =0-18). 

(d) Pressure at end of explosion. 

(e) Temperature and pressure at end of expansion. 
(/) Efficiency of the cycle. Arts. 49* 4 per cent. 
(g) Efficiency of an engine working on a Carnot 

Cycle between the same highest and lowest 
temperatures. Ans. 56 8 per cent. 

(Mech. Sc. Tripos, 1906.) 

10. Describe, with sketches, the mode of operation of an 
internal combustion engine. Explain why, in general, such 
an engine is more efficient as a heat engine than a steam 
engine of the same power. State where the various losses of 
energy occur. A gas engine of 10 brake horse-power con- 
sumes 180 cu. ft. of gas per hour, the calorific value of which 
is 690 British Thermal Units per cubic foot. Find its total 
efficiency, and give a rough estimate of the different propor- 
tions of energy lost due to the causes referred to above. 

(Mech. Sc. Tripos, Part II, 1906.) 

11. A pound of air at atmospheric pressure and 20° C. is to 
be compressed adiabatically to 10 atmospheres ; find the 
work done by the pump. The same result is arrived at 
by isothermal compression, cooling the air so that it keeps 
at 20° C, and when the pressure reaches 10 atmospheres 
it is heated at constant pressure. The specific heats of air 
are 238 and 1 69. State separately the work done upon 
and by the air and the heat taken from and given to it, all 
in foot-pounds. Ans. Adiabatic compression 65,470 ft.-lb., 
isothermal heat rejected 64,825 ft.-lb., added at constant 
pressure 92,170 ft.-lb. 

12. Criticise the " Otto " Cycle, in gas engines, from the 
point of view of (1) efficiency, (2) relation of power to weight 
on the part of the engine. In modern practice the tendency 
is to compress the mixture highly before ignition. How 
does this affect the points of your criticism ? 



CHAPTER III 

Combustion and Explosion 

Chemical Combustion — Dugald Clerk's and Grover's Early 
Experiments on Explosion in Closed Vessels — Discussion 
of Results — Increase of Specific Heats of Gases — 
Dissociation — " After-burning " — Later Experiments. 

21. Chemical Combustion. — Instances of chemical com- 
bustion are manifold. Two among the commonest are 
the burning of coal, and the oxidation of the carbon in food 
which is the source of the heat energy given out by the 
human body. In place of coal, it is possible to burn gas 
made from coal and so obtain either heat or light. In a 
gas engine cylinder gas and air are first mixed together 
and the whole mass ignited at once, so that the union is 
explosive. Useful figures to remember are that 1 lb. of 
coal on being burnt will liberate about 12,000,000 ft. -lb. of 
energy, a cubic foot of coal gas wiU liberate about 550,000 
ft. -lb., 1 lb. of petroleum about 18,000,000 ft.-lb., and 
1 lb. of petrol some 15,000,000 ft.-lb. These are very 
large amounts, and were it possible to invent a heat engine 
of 100 per cent, efficiency it is plain that a very liberal 
supply of energy would be obtainable at little cost. With 
existing engines 1 lb. of coal with potential energy equal 
to 12,000,000 ft.-lb. will only give in energy on the brake 
about 2,000,000 ft.-lb. with the best steam engines and 
4,000,000 ft.-lb. with the best gas engines, the waste energy- 
being 10,000,000 ft.-lb. and 8,000,000 ft.-lb. respectively in 
the two cases. 

The loss of 8,000,000 ft.-lb. which occurs in a gas engine 
is divided between the loss to the water in the cooling 
jacket and the loss which occurs owing to the exhaust 



COMBUSTION AND EXPLOSION 



37 



products being at a high temperature and so carry- 
ing off a large unutilized portion of the heat. The loss to 
the water jacket is the more difficult to follow in all the 
intricacies of the working cycle. The cooling jacket is 
necessary, as without it the piston and cylinder would get 
almost red hot and the engine would stop running. How 
the temperature-flow through the metal depends on the 
position of the piston in its stroke, is difficult to de- 
termine. 

22. If after a charge of gas and air has been drawn into 
a gas engine cylinder the flywheel be held so that it cannot 
move and the charge be then ignited, a rapid rise of pressure 
is recorded on the indicator. It ought, one would think, to 
be easy to calculate what this rise would be, since the 
quantity of gas and air admitted and their quality are 
easily determinable and the amount of thermal energy 
liberated therefore known. If this amount of energy be 
divided by the amount of heat required to heat the mixture 
through one degree Cent, it is clear that the resulting tem- 
perature would be ascertained, and from this it would be 

PV 

simple by the law to determine the resulting pressure. 

This has often been done, but it has always been found 
that the pressure actually obtained is only about one-half 
that calculated. Here are the actual figures obtained in 
some experiments carried out by Mr. Dugald Clerk — 



Ratio air/gas. 


Absolute Pressure 
Obtained. 


Absolute Pressure 
Calculated. 


14 


55 


110 


13 


661 


116 


12 


75 


123 


11 


76 


132 


9 


93 


161 


7 


102 


190 


6 


105 


214 


5 


106 


206 


4 


95 


196 



On an average there appears here to be a loss of as much 
as 50 per cent, of the pressure. Why is this ? 



38 THE INTERNAL COMBUSTION ENGINE 

23. Several explanations have been put forward to account 
for this loss. The most important are — 

1. The Dissociation Theory. — It is well known that 
chemical compounds such as H 2 or C0 2 dissociate at high 
temperatures into simpler gases and in so doing absorb 
heat. It has therefore been thought that at the high 
temperatures of explosion such dissociation would occur 
and the heat so absorbed might account for the missing 
50 per cent. This assumption, however, involves the 
deduction that for weak explosions, in which low pressures 
and temperatures were attained the effect should be much 
less, so that the actual pressure would form a much 
larger proportion of the calculated pressure and the con- 
verse in the case of rich mixtures. As a glance at the 
above figures will show, this, however, is not the case ; 
at the weakest mixture of 1 to 14 the missing pressure is 
50 per cent., and at the richest of 4 to 1 it is 52 per cent., 
or practically the same. This theory therefore does not 
suffice alone to account for the observed facts. 

2. The Cooling Theory. — This assumes that the cooling 
effect of the cylinder walls is so great that the pressure 
actually obtained must fall much below the ideal calculated. 
It does not explain, however, why the loss should be 
always 50 per cent, in the particular cylinder used, nor, 
moreover, does it explain why a 50 per cent, loss is found 
still to occur even when a cylinder of a different size and 
shape is chosen. So that this theory also is inadequate 
in itself to explain the observed effect. 

3. The Increasing Specific Heat Theory. — This is the 
theory advanced first by MM. Mallard and Le Chatelier 
who found as the result of their experiments that the specific 
heat of gases and particularly of C0 2 appeared to increase 
considerably with rise of temperature. The objection 
commonly alleged against this theory is that, as in the 
Dissociation Theory, it requires that a greater proportion 
of the ideal pressure should be obtained at lower tempera- 
tures than at higher, and that this is not found to be the 
case. 

4. The After -burning Theory. — This theory has chiefly 



COMBUSTION AND EXPLOSION 39 

been associated with the name of Mr. Dugald Clerk, who 
suggested that the combustion of the gas was not as rapid 
as supposed and that not all the heat was liberated before the 
moment of highest pressure. It assumes in fact that the 
gas is still burning long after the point of maximum pressure 
and that the cooling effect of the walls has therefore a 
much longer time to operate than had been generally sup- 
posed. In an actual gas engine this would mean that the 
gas would be burning right through the working stroke and 
that it must sometimes happen that unburnt gas would pass 
away in the exhaust. The objection to this theory lies in 
the fact that it has never been shown conclusively that the 
explosion is not complete at the point of highest temperature. 
Indeed the evidence is rather the other way. It is not 
usual to find the exhaust to contain more than a very few 
per cent, of unburnt gases, and Prof. Bur stall has shown 
that a complete heat balance analysis can be obtained 
without the need of any such hypothesis. 

24. Thus there are four simple theories, none of which 
appear to be sufficient in themselves to account for the 
observed loss. The difficulty is so fundamental a one that 
still further theories compounded of the above have been 
put forward. Mr. Dugald Clerk has recently suggested, as 
will be explained later at greater length, that the supposed 
50 per cent, may be accounted for on the supposition that 
part is due to the after-burning loss and part to a certain 
increase in specific heats. The author has seen no reason 
to alter the suggestion he himself made at the meeting 
of the British Association* in 1902, viz. that the so-called 
"suppression of temperature" was probably due to the 
combined action of cooling and of increase of specific heat 
on the lines suggested by the French physicists, MM. Mallard 
and Le Chatelier. It was shown that although the increase 
of specific heat left a larger proportion of loss to be 
accounted for at low temperatures than at high ones, this 
was sufficiently explained by the fact that the ignition 
period was much longer at low temperatures and so 

* The Engineer, October 10, 1902 ; Engineering, October 10. 1902. 



40 THE INTERNAL COMBUSTION ENGINE 

allowed the cooling effect to have a longer time for action 
than it would have at high temperatures. This meant 
that for weak mixtures the 50 per cent, loss was mainly 
due to cooling, for rich mixtures mainly due to increase 
of specific heat, and for intermediate mixtures was due to a 
combination of the two. 

Mr. Dugald Clerk's early experiments had consisted in 
indicating explosions of mixtures of air with Glasgow 
and Oldham gas in a closed cylinder 7 in. by 8J in. 
The indicator registered pressure p on a rotating drum 
driven at a known constant speed, so that curves were 
obtained showing the relation between p (pressure) and t 
(time) during the explosion and the subsequent cooling of 
the gas to the walls and ends of the cylinder. From the 
diagrams so obtained it was of course possible for the 
author to measure the time occupied by the explosion, 
and the subsequent rate of fall of pressure due to cooling. 
The specific heat constants were taken from the experi- 
ments of MM. Mallard and Le Chatelier as reduced by 
Prof. Bur stall in his report of the Gas Engine Research 
Committee of the Institution of Mechanical Engineers. 
That there are objections to the method of experimenting 
by which the French physicists obtained their results is 
well known. In fact Prof. Callendar has remarked : " The 
method of experiment employed was closely analogous to 
the explosion that was taking place in the gas engine itself. 
Explosive mixtures were fired in a closed cylinder 17 in. 
by 7 in., and the maximum pressure was read by means 
of a Bourdon gauge." Since the date of this paper other 
measurements have been made, and though the results 
obtained vary among themselves it may be said that on 
the whole they support the general conclusion reached 
by MM. Mallard and Le Chatelier. If the theoretical tem- 
perature of explosion is calculated from these values of 
the specific heat the difference from the observed value is 
much less. Thus a column may now be added to the table 
last given — 



COMBUSTION AND EXPLOSION 



41 







Absolute Pressure 


Absolute Pressure 


Ratio air/gas. 


Absolute Pressure 


Calculated on 


Calculated on 


Obtained. 


Constant 


Variable 






Specific Heat. 


Specific Heat. 


14 


55 


110 


83 


13 


661 


116 


86 


12 


75 


123 


901 


11 


76 


132 


95 


9 


93 


161 


107 


7 


102 


190 


121 


6 


105 


214 


131 


5 


106 


206 


127 


4 


95 


196 


123 



It will be seen that in the case of the weakest mixture the 
50 per cent, loss has been reduced to 34 per cent. , and in the case 
of the richest 52 per cent, has been reduced to 23 per cent., 
showing a step in the required direction. It is now neces- 
sary to make some allowance for the cooling and see how far 
the discrepancy still remaining may be accounted for. The 
basis on which such an allowance can be made is as follows : 
the law connecting p and t (in seconds) during cooling is 
ascertainable from the curves given by the indicator, and it is 
therefore possible to calculate what the rate of loss of heat 
energy would be at the actual maximum temperature of 
explosion. This rate is of course greater than the mean 
rate from the beginning of explosion, since initially the 
temperature of the gas was approximately atmospheric. 
Starting, however, as it does from zero and rising quickly 
to a definite maximum the mean value for the very brief 
interval of explosion may, to a first approximation, be taken 
as half the maximum. 

In view of the very considerable discrepancy to be 

accounted for — in all some 50 per cent, of the total energy 

— small variations in the constants adopted would be of 

relatively little moment. In the absence, therefore, of 

other data the writer adopted those for lighting gas as given 

in the I.M.E. Report. Change of volume due to chemical 

combustion being of very small effect is neglected. From 

31 
the cooling curve it was found that /; — , and from this 



42 THE INTERNAL COMBUSTION ENGINE 

it can be calculated that the losses due to cooling must 
be such as to give the final temperatures included in the 
following completed table — 



Air. 


Maximum Pressure on Explosion, lb. /in. 2 . 


Gas. 


On Constant 

Specific Heat 

Hyp. 


On Variable 

Specific Heat 

Hyp. 


Variable Specific As obtained 

Heat with by Mr. Dugald 
Cooling Allow- Clerk on 
ance. experiment. 


14 
13 
12 
11 

9 
7 
6 
5 
4 


110 
116 
123 
132 
161 
190 
214 
206 
196 


83 

86 

90 

95 

107 

121 

131 

127 

123 


60 

66 

70 

76 

91 

104 

115 

109 

93 


55 

66 

75 

76 

93 

102 

105 

106 

95 



The results are also shown as a curve in Fig. 8, and 
it will be admitted that the pressures so calculated in this 
way agree very well with those obtained in the experiments. 
This expression for the rate of cooling of gaseous mixtures 
enclosed in metal cylinders of stated dimensions is not 
easy to apply to the case of ordinary gas engines. First, 
because the connexion between the rate of loss of heat 
and the dimensions of the cylinder is very complicated, 
but even more because in an ordinary gas engine cylinder 
the temperature of the cylinder walls and of the piston * are 
so very different that conditions sometimes arise in which 
while the gas is being heated by the piston it is at the same 
time being cooled by the cylinder walls, a condition of affairs 
in no way analogous to that holding in the above experiments. 

25. At the time these calculations were made the only 
other well-known experiments upon the explosion of gases 
in closed vessels were those of Mr. Grover, and at the 
British Association meeting in 1903 the author presented a 
paper in which an endeavour was made to show how far the 
combined variable specific heat and cooling theory would 
.go towards explaining the very remarkable results obtained 

* See Engineering, June 27, 1902, and August 1, 1902. 



COMBUSTION AND EXPLOSION 



43 



hy Mr. Grover, which in no way resembled those obtained 
by Mr. Dugald Clerk, inasmuch as the former found much 
lower pressures and came to the unexpected conclusion 
that the retention of waste products in a gas engine 
cylinder increased the pressure of the ensuing explosion, 
an astonishing result having regard to the great care 



250 



Maximum Pressures obtained on 
explosion in a closed vessel of dif- 
ferent mixtures of coal gas and air 




10 



12 



14 



4 e 8 

Ratio Air/Gas. 

(Curve A: Maximum pressure on Const: Sp: Heat Hypothesis. 

XcurveB: " " » Var: " 

{CurveC: " « " " •• " » with Cooling 

Points marked O are Dugald Clerk's Results. [Allowance 

Fig. 8. 



taken by most gas engine manufacturers to sweep out 
the greatest possible amount of the products of old ex- 
plosions. The great difference between the maximum 
pressures obtained by Mr. Dugald Clerk and Mr. Grover is 
illustrated in the figure that follows.* 

It was Mr. Grover's idea not only to measure the pres- 



* See also Engineering, September 18, 1903. 



44 THE INTERNAL COMBUSTION ENGINE 

sures produced by various richnesses of mixture of coal- 
gas and air, but to investigate whether the resultant pres- 
sure on explosion was affected by replacing the air in 
excess of that calculated as chemically necessary for com- 
plete combustion by a portion of the burnt products of 
the previous explosion. It appears from Mr. Grover's 
account of the experiments that he had an iron cylinder 
of one cubic foot capacity, and that in each series of ex- 
periments the volume of the coal gas admitted was kept 
constant and the cylinder was then filled with a mixture 
of air and waste products in various proportions. This 
was done in each series by filling the cylinder with water, 
and allowing gas to enter whilst a known volume of water 
was run out. Thus after an explosion, water was allowed 
to pass into the cylinder until all but the required volume 
of burnt products had been forced out ; so that if it were 
desired that no burnt products should be left, the cylinder 
would be completely filled with water, but if, say, 50 per 
cent, of the volume of the cylinder was required to con- 
tain burnt products, the water would only be permitted 
to rise half-way up the cylinder. 

The pressure was recorded in the customary manner on 
a rotating drum, but very few of the curves are given in 
the published account of the experiments, and it is there- 
fore difficult to make a very exact comparison between 
the time rate of fall of the pressure after explosion in Mr. 
Grover's experiments (using, of course, those experiments 
in which no burnt products were admitted) with those of 
Mr. Dugald Clerk. However, so far as the curves can be 
examined, they show for the same pressures almost exactly 
the same rate of fall, a result which is the less unexpected, 
as the diameters of the two cylinders appear to have been 
nearly equal. It is not difficult to calculate what the 
ideal maximum temperature and corresponding pressure 
of explosion would be were there a variable specific heat, 
but no cooling of the gas by the walls, and when this has 
been done, it may be compared with the pressure found 
experimentally. The following table shows the result of 
such a calculation — 



COMBUSTION AND EXPLOSION 
Table I. 



45 



Ratio of Gas 
to Air. 


Pressure 
Observed. 


Pressure 
Calculated. 


Corresponding 

Difference in 

Energy (Approx.). 




(Absol.) 


(Absol.) 


Ft. -lb. 


15 


31 


73 


21,000 


14 


39 


76 


19,000 


13 


46 


79 


18,000 


12 


51 


83 


18,000 


10 


63 


92 


18,000 


8 


77 


104 


18,000 


6 


77 


119 


31,000 



In this table there is also given the difference in the 



Max imam pressures obtained 
on Explosion in a closed vessel of 
different mixtures of Air and 
Coal-gas, by different observers. 




$7 8 9 10 II 12 12 14 

Fig. 9. — Explosion curves showing much lower pressures obtained by Mr. 
Grover than by other observers. 

heat energy between the gas at this temperature and at 
the actual temperature attained. These amounts were 
found in Mr. Dugald Clerk's experiments to be of almost 
exactly the calculated amount that would be expected to be 
lost to the walls and ends of the cylinder by cooling during 
the time of explosion. Such a hypothesis, however, docs 



46 THE INTERNAL COMBUSTION ENGINE 

not fit Mr. Gr over's results. Nor is it to be expected — 
having regard to the extreme divergence between their 
results — that both would be susceptible to the same treat- 
ment. 

The above curves show the actual pressures plotted with 
respect to richness of mixture for the experiments of both 
investigators. It is seen that Mr. Grover's curve lies far 
below Mr. Dugald Clerk's. This cannot be due entirely 
to the different cylinder volumes used (317 and 1,728 cubic 
inches), or to differences in the chemical constitution of the 
gases, because, as will be seen from th eintermediate curve „ 
there is little disagreement between the results obtained 
by Mr. Dugald Clerk and Mr. Douglas, although the 
results * obtained by the latter were for gases enclosed, not 
in iron cylinders, but in a eudiometer tube. If the use 
of a eudiometer does not produce results more different 
from Mr. Dugald Clerk's than this, the presumption 
certainly is that some factor must have entered into Mr. 
Grover's experiments which has entirely masked his results. 
A suggestion as to what this factor could be has been made 
by Mr. Grover himself, for in describing one experiment 
he says : " The difference is no doubt due to the fact that 
water was present on the walls of the cylinder ; " but Mr. 
Grover did not consider apparently that this presence of 
water affected his conclusions on the subject generally — 
conclusions which are set forth on pp. 231 and 232 of his 
Modern Gas and Oil Engines. In his paper the author had 
no hesitation in attributing not only the discrepancy in the 
heat balance-sheet of the experiments, but also the extra- 
ordinary results obtained in the allied series of experiments, 
in which burnt products were present, to the effect of a 
water film. 

There is, of course, a limit to the quantity of water 
which could adhere to the walls of the cylinder, and it is 
necessary to see whether the required amount is what 
could reasonably be expected to exist. The average loss 
of energy given in column 4 of the table on p. 45 is about 

* See The Engineer, April 22, 1887, and November 7, 1902. 



COMBUSTION AND EXPLOSION 47 

20,000 foot-pounds, and considering the average energy 
given to 1 lb. of water to raise it from atmospheric tem- 
perature to superheated steam at the average maximum 
temperature, as about 750 Cent, heat units, it follows that 
the weight of water required equals 0191 lb. This would 
occupy a space of about 53 cubic inch, and in a cylinder 
of the dimensions used, a film of water 2~oVo m * 
thick would be sufficient to account for this. So that 
there is no difficulty in accounting for the presence of a 
sufficient quantity of water. It is interesting to assume 
the presence of this small quantity of water, and to- 
trace its effect throughout the whole explosion. As it 
happens, the actual calculations involved are a little 
tedious ; as the temperature rises the vapour tension of the 
water increases until a time comes when steam is given 
off ; this, of course, does not occur at the ordinary boiling 
point, as by this time the gases in the cylinder will be at 
a pressure in excess of the atmospheric pressure. The 
temperature still rises, but so long as there is any water 
left, the steam is saturated and the temperature of the 
gases must therefore keep step with the pressure- — that is, 
if the contents of the cylinder are assumed to be all at 
the same temperature at the same time, and without such 
an assumption calculation is out of the question. Next y 
a point is reached at which the steam begins to be super- 
heated ; further, the formation of a volume of steam has 
meant the compression of the gases in the cylinder so 
altering the relation between the pressure and the tem- 
perature, and bringing a further complication into the 
matter. It therefore becomes a matter for careful treat- 
ment to draw out a temperature entropy curve for the 
whole. (Perhaps the simplest way is to think of the gas 
and the water stuff being kept separated by a thin dia- 
phragm, but kept at the same temperature at the same 
time.) To show the result of the steam compressing the 
gas a simple example may be taken, in which the weight 
of the film of water is three-eighths of the weight of the 
gaseous mixture, and in which both start at atmospheric 
temperature and pressure. When the pressure (absolute) 



43 THE INTERNAL COMBUSTION EXGIXE 

amounted to 33 lb. per square inch, a calculation made in 
the absence of the knowledge of the presence of a water 
film would give a temperature of 254 c Cent., whereas the 
real temperature would be 124" Cent., a very different 
result. A further calculation with the same amount of 
water present shows that a pressure of 60 lb. per square inch 
would be attained on explosion, whereas under the same 
circumstances, but in the absence of the water film, a pressure 
of 100 lb. per square inch would have been attained. 

The curves and tables above given are for the experiments 
made by Mr. Grover with mixtures of coal-gas and air only. 
Xo burnt products were present. When burnt products 
were admitted, very remarkable results were observed. 
Taking the first series * of experiments (in which the volume 
of coal-gas was one-sixteenth of the total volume of the 
cylinder), when there were no burnt products present, the 
pressure recorded was 16 lb. per square inch above the 
atmosphere. When burnt products were present to the 
extent of 13-3 of the volume of the cylinder the pressure 
rose to 35 lb. per square inch above the atmosphere. 

Xow it will be observed that in the experiment in which 
burnt products were absent, the whole of the interior of 
the cylinder must have been wetted by the water used in 
the measurement of the volumes, and on the other hand. 
when burnt products were present, the water only rose 
up to two-thirds of the height of the cylinder. Hence, 
in the second experiment, one-third of the exposed surface 
was dry, and would therefore cool the gases in the manner 
already observed in Mr. Dugald Clerk's experiments, whilst 
the remaining two-thirds of the surface was covered by a 
water film which would, as explained above, absorb much 
of the heat energy of the gas before it was evaporated. 
When the whole surface was wet. only 16 lb. per square 
inch was registered : and when one-third was dry and two- 
thirds wet the pressure rose to 35 lb. per square inch. 

The author concluded that the presence of a water film 
of varying extent was a sufficient explanation of the very 

* Graver's Modern Gas and Oil Engines. 1902 edition, p. 233. 



COMBUSTION AND EXPLOSION 49 

curious results obtained by Mr. Grover. The hypothesis 
upon which this explanation was made leads, however, to 
a further deduction, the truth of which it remained to investi- 
gate. Mr. Dugald Clerk's curves showed that the rate of 
loss of heat energy to the walls increased much more rapidly 
than the temperature of the gas ; in fact, the rate of loss 
was about proportional to the third power of the absolute 
temperature. It followed, therefore, that in the richer 
mixtures in which higher temperatures would be attained, 
the increase in the loss owing to the increased cooling effect 
of the fraction of dry wall exposed, would be much greater 
than the saving due to the water film only covering two- 
thirds instead of the whole of the surface, and that in con- 
sequence, for the richer mixtures, the apparent effect of 
burnt products in increasing the resultant pressure would 
be much diminished, if not extinguished altogether. Now 
this was precisely what had been found by Mr. Grover to be 
the case. When the ratio of volume of gas to volume of 
cylinder was 1 to 13, the effect of burnt products was 
to increase the pressure from 36 lb. to 43 lb. per square 
inch — a far smaller increase than before ; and when the 
ratio was increased to 1 to 9, the effect was quite wiped 
out, and the pressure fell almost immediately the burnt 
products were admitted. It is not, however, surprising that 
the unexpected results found by Mr. Grover should be 
capable of being accounted for without it being necessary 
to assume that the burnt products really would increase 
the pressure under such conditions as usually hold in a 
gas engine cylinder. The only reason why burnt products 
might exercise any such tendency would lie in their having 
a smaller specific heat than the explosive mixture, which 
is not the case. In practice it has been found that the 
presence of burnt products in the explosive mixture has any- 
thing but a good influence on the economy of gas engines. 
The results of this investigation into these early experi- 
ments on gaseous explosion, made it appear to the author 
that it could not be wise, in gas engine calculations, to 
assume the constancy of the specific heats of the working 
gases. Indeed, that it might be greatly doubted whether such 

E 



50 THE INTERNAL COMBUSTION ENGINE 

calculations could be regarded as of permanent value, unless 
an alternative calculation founded on the basis of a variable 
specific heat was also given. In his calculations for the 
Institution of Mechanical Engineers, a linear equation con- 
necting specific heat with temperature was used by Professor 
Burstall,* and it was found to lead to consistent results. 

26. Exercise. — What percentage of dissociation of C0 2 to CO and O 
at, say, 1,700° C. would suffice to mask the effect of the rise of true 
specific heat ? To solve this it must be remembered that if we have 
44 kg. of C0 2 and 1 per cent, of it is dissociated to CO and O it would 
require an absorption of heat equal to 682 calories {see Chapter VI. 
as to this) ; also that the rise in true specific heat would probably 
be of the order 0-2, corresponding to an additional absorption of 

heat of x - 2 calories per kg. of C0 2 or x 8"8 calories for 

— — 

44 kg. which comes to 7,480 calories. 

It follows from this that 1 per cent, of dissociation would cause the 
apparent specific heat to come out at about 11 per cent, higher than 
the value of the true specific heat. In connexion with this it may 
be remembered that Langen could detect no evidence of dissociation 
up to 1,700° C. in his explosion experiments. On the other hand 
it is clear that 1 per cent, of dissociation could materially affect 
specific heat measurements and that 10 per cent, would be near to 
doubling the apparent rate of increase of specific heat. 

27. Later Experiments. — So much for the earlier experi- 
ments on the combustion of gas and air ; the writer has 
stated how a number of years ago he came to the con- 
clusion that increase of specific heat was the determining 
cause in the apparent " suppression of heat," and a further 
illustration will be given in the following chapter. In 
addition to the early experiments of Dugald Clerk and 
Grover, some work in the same direction was done at the Mas- 
sachusetts Institute of Technology, but it does not appear 

* Professor Burstall appears to have made his calculations thus : — 
The exact amount of energy liberated on explosion was known. The 
variable specific heat allowed of three-fourths of the energy being 
accounted for. It was then assumed that the remainder was lost 
to the water-jacket during explosion, the amount so lost during 
expansion being taken as the difference between the work done and 
the change in the internal energy of the gas. The same process 
being carried out during the compression, the net loss to the jacket 
per cycle could be calculated. This should agree with the observed 
loss owing to heating of the water-jacket, and in practice a satis- 
factory agreement was observed. 



COMBUSTION AND EXPLOSION 51 

that any definite conclusions were drawn therefrom. Later 
experiments have been made by Mr. Dugald Clerk, Professor 
Hopkinson and Messrs Bairstow and Alexander. Taking 
the last first : Messrs. Bairstow and Alexander's experi- 
ments were made on mixtures of London coal-gas and air 
in a cylinder 18 in. long and 10 in. in diameter, pressures 
were indicated on a rotating drum, and the results of the 
investigations were communicated to the Southport meeting 
of the British Association in 1903. The conclusions reached 
were not, however, in accordance with the still later experi- 
mental results obtained by Mr. Dugald Clerk and Professor 
Hopkinson, and it is not necessary to say more than that 
Messrs. Bairstow and Alexander considered that " at the 
high pressure reached by the best explosive mixtures, the 
loss due to cooling is less than the errors of observation, the 
calculated and actual values differing only on account of 
dissociation. At the lower pressures given by the weaker 
mixtures the whole of the loss of pressure is due to cooling. 
This shows a temperature of about 1,200° Cent, before 
carbon dioxide begins to dissociate." It will be seen that 
these conclusions are different from those which the 
author arrived at as the result of an investigation of other 
experimental results. In a paper communicated to the 
Royal Society two years later the same authors repeat their 
conclusions in more explicit language : — "Mixtures of coal- 
gas and air are not inflammable until the volume of coal-gas 
is greater than one-seventeenth of the combined volumes. 
Only a very small fraction of the gas then burns, the amount 
rapidly increasing with increased richness of the mixture 
until the coal-gas is one-twelfth of the total volume. The 
least inflammable of the constituents then burns, and com- 
bustion becomes and remains complete so long as air is in 
excess. In these latter cases it is still probable that the 
constituents burn successively and not simultaneously. 
The hypothesis of a specific heat increasing with tempera- 
ture is not supported by direct experiment, and cannot be 
proved by any work on the pressures produced by explosion, 
the authors believing that a proof would require the mea- 
surement of temperature. Direct experiments by Deville 



52 THE INTERNAL COMBUSTION ENGINE 

at temperatures below 1,400° C. have shown that both 
steam and carbon dioxide are partially decomposed, and 
this dissociation is therefore taken by us as the sole explana- 
tion of the difference between the pressures calculated for 
explosions in a closed vessel and those actually obtained." 
As will be seen later, Professor Hopkinson has since made 
direct measurements of the temperatures. 

28. In 1906 Mr. Dugald Clerk communicated to the Royal 
Society the results of some experiments he had made by a new 
method. It had often been remarked that the expansion 
curve on gas engine diagrams was found to lie above the 
adiabatic line, and many deductions had been made there- 
from. Mr. Dugald Clerk's new method of experiment consisted 
in running a gas engine under the ordinary standard condi- 
tions and then at a given moment preventing the exhaust and 
inlet valves from opening, and at the same time taking a series 
of indicator diagrams. These diagrams showed a number 
of expansion and compression curves with the pressures 
gradually falling as the gas cooled. Fig. 10 is a repre- 
sentation of a series of curves so obtained. From the shape 




Fig. 10. — Dugald Clerk's " Zig-Zag " curves. 1/9 mixture 



of these curves it was possible to make refined calculations 
as to what was occurring to the gas in the cylinder. The 
average temperature must clearly diminish owing to the 
effect of the water-cooled walls, whilst if combustion con- 
tinues after the point of maximum temperature an effect 



COMBUSTION AND EXPLOSION 53 

must be observed in an apparent lessening or even reversing 
of the cooling action. This is merely a general idea of the 
method of experiment and calculation. In its details it 
consists of a series of secondary approximations by means 
of secondary curves, in order to ascertain what the apparent 
specific heat of the mixture must be at any given tempera- 
ture and at any point of the stroke. In these calculations 
the work done by the flywheel on the gas is allowed for. 
The values of the apparent specific heats at 1,000° C. and 
1,500° C. as found by this well-known authority agree 
fairly well with the values calculated from the Mallard 
and Le Chatelier formulae, viz. — 

For C0 2 ; O v =0 1423+0 0000834? T 
H 2 0; O,=0-3116- r -0-0001822 T 
N 2 ; C v =0-I7l +00000215T 
2 ; C v =0-150 -fO-0000188T 

but as regards the " suppression of heat " controversy his 
conclusions are : 

" (1) The apparent specific heat of the working fluid 
of the internal combustion engine (consisting mainly of 
nitrogen, carbon dioxide, steam and oxygen), when cal- 
culated from the first three-tenths of the engine stroke, 
undoubtedly increases between the observed temperatures 
300° C. and 1,500° C, but tends to a limit at the upper 
temperature. 

" (2) The apparent change in specific heat is not entirely 
due to a real change in specific heat, but requires in addition 
continuing combustion to account for all the facts. 

" (3) The rate of heat-flow from the working fluid to 
its enclosing walls for equal temperature differences varies 
throughout the stroke. Increased heat-flow accompanies 
increased mean density. 

" (4) The mean temperature of the inner surface of the 
enclosing walls varies with the portion of the stroke ex- 
amined from 190° C. for whole stroke to 400° C. for first 
three-tenth stroke under working conditions at full load. 
These mean temperatures, however, are not the highest 
mean temperatures reached by the walls. 



54 THE INTERNAL COMBUSTION ENGINE 

" (5) The heat distribution during the operation of the 
working fluid can be determined with approximate accuracy 
from the apparent specific heat values and heat -flow values 
obtained from the diagram only." 

It will be seen that these conclusions show some modifica- 
tion of the position taken up earlier by Mr. Dugald Clerk, 
although he still considers that combustion is not complete 
at the point of highest pressure. To quote his own words : 
" When the writer began the present investigation he believed 
that these phenomena of slower chemical action furnished 
a complete explanation of the discord between the theo- 
retical and observed results, and that there was no need to 
assume any considerable dissociation or variation of specific 
heat of the products of combustion. These experiments, 
however, appear to him to indicate real change of specific 
heat as well as continuation of combustion. The experi- 
ments do not exclude dissociation or any other molecular 
change which by requiring the performance of work would 
change specific heat. It appears improbable, however, 
that dissociation should be material for temperatures so low 
as 600° C. It is not usual to suppose that either carbon 
dioxide or steam can be decomposed to any sensible extent 
at such temperatures." 

In giving due weight to the conclusions so reached, it 
must not be forgotten that the method is essentially one of 
small differences, and the experimenter himself admits 
that the whole of his deductions could be greatly affected 
by an inaccuracy in the indicator curves of one-fiftieth 
part of an inch and affected not a little by an error of 
y^-q inch. Having regard to the circumstances, in respect 
of inertia and time-lag, in which even the best indicator 
works, it will be realized that too much reliance must not 
be placed upon the results obtained. There is no doubt 
that Mr. Dugald Clerk used an exceptionally accurate 
instrument and treated his diagrams with great skill ; as 
a piece of difficult experimental work he is certainly entitled 
to congratulation on the results achieved. The actual 
figures obtained by Mr. Dugald Clerk for the specific heat 
are shown in the following two tables. 



COMBUSTION AND EXPLOSION 



55 



Table I. — Apparent Specific Heats (Instantaneous) in Foot- 
pounds per Cubic Foot or Working Fluid at 0° C. and 
760 mm. 



Temperature. 


Specific Heat at 
Constant Volume. 


Temperature. 


Specific Heat at 
Constant Volume. 


Degrees C. 


Ft. -lb. 


Degrees C. 


Ft. -lb. 





19-6 


800 


26-2 


100 


20-9 


900 


26-6 


200 


220 


1,000 


26-8 


300 


230 


1,100 


270 


400 


23-9 


1,200 


27-2 


500 


24-8 


1,300 


27-3 


600 


25-2 


1,400 


27-35 


700 


25-7 


1,500 


27-45 



Table II. — Mean Apparent Specific Heats in Foot-pounds per 
Cubic Foot of Working Fluid at 0° C. and 760 mm. 



Temperature. 


Specific Heat at 
Constant Volume. 


Temperature. 


Specific Heat at 
Constant Volume. 


Degrees C. 


Ft.-lb. 


Degrees C. 


Ft.-lb. 


0—100 


20-3 


0—900 


239 


0—200 


20-9 


0—900 


241 


0—300 


21-4 


0—1,100 


24-4 


6—400 


21-9 


0—1,200 


24-6 


0—500 


22-4 


0—1,300 


24-8 


0—600 


22-8 


0—1,400 


250 


0—700 


232 


0—1,500 


25-2 


0—800 


23-6 


— 


— 



It will be observed from the above that although the 
apparent specific heat was found to increase with rise of 
temperature, it tended towards a limiting value. The 
increase found for the first 500° C. was far more than for 
the last 500°. This conclusion is not in accord with the 
experiments of other workers. 

29. The last series of experiments of this kind to be 
described are those of Prof. Hopkinson, communicated to 
the Royal Society in 1906. These were explosion experi- 
ments in a closed vessel as shown in Fig. 11. A is the 
sparking point, B, C and D are platinum thermometers. 



56 THE INTERNAL COMBUSTION ENGINE 

Thermometer B is practically at the centre of the vessel, 
C is about 30 cm. distant from the spark and D is about 
1 cm. from the walls of the vessel. A record of the pressure 
was taken on the same drum as that upon which the tem- 




Fig. 11. — Professor Hopkinson's Gas Explosion Apparatus. 



peratures were electrically recorded. The indicator was 
very simple, consisting as it did of a piston controlled by a 
flat steel spring held at the two ends. As the spring was 
deflected a mirror tilted and so threw a beam of light on to 
the moving film. The period of the instrument was about 
3--J-Q sec. Fig. 12 shows the kind of result obtained when 
plotted out. The following table serves also to show the 
actual indications recorded by the electric thermometer 
placed at the centre of the vessel : — 



COMBUSTION AND EXPLOSION 



57 



Time. 


Resistance. 


Rise of Resistance. 


Temperature in 


Sees. 


Ohms. 


Ohms. 


Degrees C. 


0-008 


2205 


12-4 


560 


0024 


30-3 


20-7 


995 


041 


32-7 


231 


1,135 


0057 


331 


23-5 


1,165 


0074 


33-1 


23-5 


1,165 


009 


340 


24-4 


1,225 


0107 


34-5 


24-9 


1,260 


0123 


34-7 


251 


1,275 


0140 


34-7 


251 


1,275 


0173 


36-6 


270 


1,400 


0-26 


wire melts 


— 


1,710 



An investigation had also to be made into the question of 
the existence of a time and temperature lag in the tem- 



2.500' 



2,006 



1,500 




1.200 

i.o od 



•05 01 015 0-2 0-25 03 Sees. 

Time . 

Fig. 12. 



perature recorded by the thin platinum wire. A theoretical 
investigation of this problem was carried out by the writer 
some years ago when contemplating undertaking a series 
of experiments similar to those of Prof. Hopkinson, but 



58 THE INTERNAL COMBUSTION ENGINE 

which had to be set on one side owing to the pressure of 
other work. It suffices to say here that Prof. Hopkinson 
found the temperature of the wire to lag materially behind- 
that of the gas when the latter was changing rapidly. To 
test this, wires of two different thicknesses were used, viz. 
1 ^qq in. and ^^q-q in. respectively, and by a comparison of 
the results obtained Prof. Hopkinson was able to find the 
amount of the correction which he considered it necessary 
to employ. 

The most important of the conclusions reached by this 
experimenter, who, he tells his readers, carried out these 
experiments, largely " with the object of finding the cause 
of the so-called 'suppression of heat' in explosions," 
is that his experiments appear to prove that even in the 
weakest mixtures combustion, when once initiated at any 
point, is almost instantaneously complete. Moreover, he 
adds, they show that the specific heat of the products is 
very much greater at high temperatures than at low, and 
the extent of the difference seems to justify the view that 
it is the main reason of the so-called " suppression of heat." 
He adds that this rise in the specific heat is consistent with 
direct observations of that constant for C0 2 , which have 
been made up to about 800° C, and which prove that it 
increases considerably. 

30. In addition to these conclusions Prof. Hopkinson 
found certain differences in the temperature of the gas in 
different parts of the vessel, and this supports the results 
obtained by Prof. Burstall in his gas engine trials for the 
Institution of Mechanical Engineers. In experimenting 
with a rich mixture (air /gas =9) Professor Hopkinson found 
that at the moment of maximum pressure the distribution 
of temperature in his vessel was roughly as follows — 
Mean temperature (inferred from pressure) 1,600° C. 

(a) Centre near spark .... 1,900° C. 

(b) 10 cm. within the waU (C, Fig. 11) . 1,700° C. 

(c) 1 cm. fromwaU at end (D,Fig. 11) 1,100 to 1,300° C. 

(d) 1 cm. from wall at side . . . 850° C. 
It is explained that " at points a, b and c the gases can 

liave lost but little heat at this time, and the differences of 



COMBUSTION AND EXPLOSION 59 

temperature are almost wholly due to the different treatment 
of the gas at different places. At (a) it has been burnt 
nearly at atmospheric pressure, and compressed after burning 
to about 6 \ atmospheres absolute, while at (c) it has been 
first compressed to about six atmospheres as in a gas engine, 
and then ignited without any subsequent compression. 
At the point (d) much heat has been lost, since this is the 
first point on the wall reached by the flame ; the gas here 
is ignited when the pressure is about two atmospheres, its 
temperature rises instantly to 1,300° C. and at once begins 
to fall." 

In experiments on a weak mixture of twelve volumes of 
air to one of gas the explosion was affected very greatly by 
the convection current setup, owing to the ignited gas being 
lighter and rising through the vessel. In the rich mixture 
this could not happen to the same extent as the maximum 
pressure was reached about a quarter of a second after firing, 
whilst with the weak mixture the interval was two and a half 
seconds and so the time for convection was much longer. 
The experimenter recorded that " a few centimetres below 
the spark the temperature will rise rapidly and then fall ; 
the flame reaches the wire, and is then carried upward and 
away from it, the wire being cooled by the current of cold, 
unburnt gas which follows in the wake of the ascending 
flame. About one second after ignition, and while the 
pressure is still less than 10 lb. above atmosphere, the upper 
part of the vessel is filled with burnt gas which is in contact 
with, and losing heat to, the upper half of the walls." The 
lower half of the gas is therefore burnt last. Finally it 
may be recorded that Professor Hopkinson in comparing 
the behaviour of rich and poor mixtures says : "It is safe 
to assume in dealing with a 12/1 mixture that one-fifth 
of a second after maximum pressure (when the loss of pres- 
sure by cooling is still less than 5 per cent.) there is present 
in the cylinder a mass of C0 2 , H 2 0, and inert gas in complete 
chemical equilibrium. In the 9/1 mixture this state is, of 
course, attained very much sooner. The difference in the 
behaviour of the weak and strong mixtures is wholly due 
to the very slow propagation of flame in the former ; in a 



60 THE INTERNAL COMBUSTION ENGINE 

9/1 mixture the flame seems to travel about ten times as 
fast as in the 12/1 mixture." 

The writer naturally has satisfaction in finding that the 
results of the latest and most complete experiments so far 
made in this subject should go so far to confirm the conclusions 
to which he came by a theoretic process of reasoning many 
years ago. In the following chapter an investigation will 
be made as to the modifications in the customary gas engine 
formulae which the adoption of variable specific heat values 
will require. 

EXAMPLES. 

1. Answer only one of the following, either A or B : — 

A. Change into horse-power the rates of conversion of 
chemical energy by combustion of the following : — 1 lb. of 
kerosene per hour ; 1 cubic foot of coal-gas per hour ; 
1 cubic foot of Dowson gas per hour ; 1 lb. of coal per hour. 
The calorific powers are, in Fahrenheit pound heat units 
1 lb. of kerosene, 22,000 ; 1 lb. coal, 15,000 ; 1 cubic foot 
of coal-gas, 700 ; 1 cubic foot of Dowson gas, 160. 

B. Using the calorific powers given above, calculate the 
efficiencies of — 

(a) A large good condensing engine, using 2 lb. of coal 

per brake horse-power-hour. Ans. 8" 47 per cent. 

(b) A gas engine using 26 cubic feet of coal-gas per 

brake horse-power-hour. Ans. 14 per cent. 

(c) The Diesel oil engine which is said to use 0*56 lb. 

of kerosene per brake horse-power-hour. Ans. 
20-6 per cent. (B. of E., 1899.) 

2. State the following amounts of energy in foot-pounds — 

(a) A weight of 35 tons may fall vertically 15 feet. 

(b) The Kinetic Energy of a projectile of 60 lb. moving 

2,000 feet per second. 

(c) The Calorific Energy of 1 lb. of coal, 8,500 Centi- 

grade pound heat units. 

(d) 30 lb. of water raised from 40° F. to 103° F. 

(e) One horse-power-hour. 

(/) One kilowatt-hour. (B. of E., 1906.) 



COMBUSTION AND EXPLOSION 61 

3 The temperatures on two sides of an iron plate 5 in. 
thick differ by 10 Centigrade degrees, how much heat (in 
Centigrade pound water units) passes per square foot per 
second ? The conductivity of iron is 0*18 in C.G.S. units 
(Centigrade gramme water units). (B. of E., 1906.) 

4. A pound of oil contains 85 lb. of carbon and 01 5 lb. 
of hydrogen. What weight of oxygen is sufficient to pro- 
duce C0 2 and H 2 by combustion ? Take the atomic 
weights of C, 12 ; of 0, 16 ; of H, 1. If 1 lb. of oxygen is 
contained in 4-35 lb. of air, how many pounds of air are 
needed for complete combustion ? Ans. 3*47 lb. and 151 lb. 

5. Describe any form of coal calorimeter and its method 
of use. 

A sample of coal is tested in such a calorimeter, and the 

following data are observed. Determine its calorific value. 

Weight of coal .... 

Weight of water 

Water equivalent of apparatus . 
Initial temperature 
Final temperature 
Time of rise . . . 

Time of temperature to fall from 57 3° 

to 55-3° 30 min. 

Ans. 13,100 B.T.U. per lb. 

(Mech. Sc. Tripos, Part II, 1906.) 

6. A gas engine exhausts into a calorimeter in which 
the gases are cooled by having water sprayed into them. 
The temperature of the gases after passing the calorimeter 
is 150° F. Assuming them to be then saturated with 
moisture, find the change of volume after they have been 
further cooled to 60° F. How would you find the heat 
evolved by the gases in so cooling ? 

(Mech. Sc. Tripos, Part I, 1905.) 



. 0015 1b. 


. 121b. 


. 3 1b. 


. 50 1°F. 


. 62-5° F. 


10 min. 



CHAPTER IV 

Thermodynamics 

First Law of Theemodyxamics — Second Law — Rates of Coollng- 
of Gases — Form of Adiabatic with Variable Specific 
Heats — Analysis of Certain Experiments — Measurement 
of Cylinder Temperature — Revision of " Atr Standard " 
— Later Measurements of Specific Heat — Flow of Heat 
through Metal Walls of Cylinder — Appendix. 

31. The applications of thermodynamics to the study of 
gas engine problems are numerous and varied. The earlier 
chapter on the efficiency of cycles of operation will have 
afforded illustration of this, but it is proposed now to devote 
further attention to the matter. 

Specific heat has already been defined, and indeed most 
students, from their work on other subjects, will be farniliar 
in advance with calculations into which it enters. The 
relation between the p, V and T of a perfect gas has been 

vV 

given as — — =i2, and in thermodynamic calculations it is 

generally necessary to assume all gases to follow this law, 
which it happens fortunately they very nearly do. A defin- 
ition has been given in an earlier chapter of entropy or (p. 
A new property has now to be introduced. It is called the 
Intrinsic Energy, and is known commonly as E. 

The Intrinsic Energy is the total energy actually in the 
substance at any moment. Thus if heat fi" is given to a body 
and mechanical work W is done by the body, then the gain in 
intrinsic energy is H — W, and this maybe positive or nega- 
tive. So that E=H — W, or if small charges are dealt with, 
as is the most cautious procedure, it becomes SE=SH — SW. 

32. The point has now come at which the reader may be 
introduced to the two laws of thermodynamics — 

1st Laic. — The E in the gas is always the same when the 
gas returns to the same state ; E can be calculated at once 
if two of the variables p, V or T are known. 



THERMODYNAMICS 63 

2nd Law. — The <p of the gas is always the same when the 
gas returns to the same state ; (p can therefore be calculated 
if the state be known. 

From these two laws a superstructure can be raised of 
deductions and theorems which are useful in the solution 
of gas engine problems. 

33. In a perfect gas, E depends upon the temperature only, 
and it is equal to the product of the temperature and the 
specific heat at constant volume. Whether the temperature 
be reckoned from 0° C. or from the absolute zero is usually of 
little importance, as it is only the change in E that has to be 
considered, and not its absolute amount ; so that 

SE=CJT- 
and since $E = SH — S W 

therefore SH=C V .ST+SW 

or the addition of heat, SH, to the gas is balanced by C V .ST, 
the gas in intrinsic energy, plus the external work done. 
The latter can also be written as p.$V, thus giving 
$H=CjT+p.SV 

or jw = c °-w +p •• (1) 

In any changes that occur to a gas it is therefore possible 

to find the rate of change of H for unit change of volume by 

adding together the two terms on the right of the equation. 

SH 
The ratio -=; — , which it will be observed is of the nature 

oV 

of a pressure, is a very important quantity in gas engine 
expansion and compression curves, and it is necessary to find 
an expression for it, which could be more quickly dealt with 
than equation (1). 

It is required to get into a more workable form the equa- 
tion. 

m st, 

Jv =c *Sv + ' p 

Since ?— =i? 

T 

T= pv 

E 



64 THE INTERNAL COMBUSTION ENGINE 



and 



substitute and 



R \ P dV) 



dT 
~d~V 

dH^_ C v 
dV 5" 

Tbut R= C p — C v and the ratio of C p to C v is y,( — — — y \ 

V C v J 

dp 



p+yd p 



Therefore 



or 



or 



dH 

dV 

dH^ 

~dV 

dH_ 
dV 






v+v 



dV 



+p 



r 



^ +v w^ 



i 

7-1 






(2) 



This is found to be an easy expression to work with, and 

dH 
it gives — — in terms of pressure units. 

34. The following table, part of which was calculated 
from a gas engine indicator card by the writer for Professor 
Perry's book on the Steam Engine, affords an illustration of 
the use of the formula (2) — y in this case was taken at the 
value 1-385. 





V. 




&- 


Average 


Average 


dH 




P- 


T^ 


V. 


V- 


dV. 


Compres- J 


25 


14-7 


-0-96 


22-5 


171 


5-92 


20 


19-5 


-1-70 


17 


24-6 


13-8 


sion 1 


14 


29-7 


-3-88 


12 


37-5 


14-6 


10 


45-2 












10 


45-2 


173 


101 


62-4 


4,760 




10-2 


79-7 


218 


10-3 


101-5 


6,210 




10-4 


123-2 


173 


10-5 


140-4 


5,230 




10-6 


157-7 


120 


10-7 


169-7 


3,930 




10-8 


181-7 


33 


10-9 


184-9 


1,590 


Explosion 


110 


188-2 


-22 


11-5 


177-2 


-20-8 


and / 


120 


166-2 


-20 


12-5 


156-2 


-85-8 


Expansion 


13 


146-2 


-14-8 


14 


131-5 


-64-9 




15 


116-7 


-10-5 


16 


106-2 


-54 5 




17 


95 7 


-7-5 


18 


88-2 


-33-8 




19 


80-7 


-60 


20 


74-7 


-41-5 




21 


68-7 


-5-0 


22 


63-7 


-571 


I 


23 


58-7 











THERMODYNAMICS 



65 



6,000 . 








' 




5,000. 








O 


Curve showing ~jy 






i 




during Explosion 








) 


and Expansion. 


4,000. 


dH 
dV 






o 




3,000. 












2,000. 








< > 




1,0 00. 












o* 


Ze ro 


Line 








-1,000 








V 


_o o o— — o— -o 



10 



20 



25 



Fig. 



13.— Curve of S and V. 

dV 



It shows how the working stuff receives 



heat during explosive combustion and how it afterwards loses heat to 
the walls. Note the position of the zero line ; values below it are of 
course negative. 

F 



66 " THE INTERNAL COMBUSTION ENGINE 

These figures are plotted to scale in Fig. 13. It will be 

JTT 

noted from the table that during compression is positive 

dV 

in every case, showing that dH and d V must be of the same 
sign. As V is decreasing during compression d V must be 
negative and therefore dH also. So that during compres- 
sion the gas is losing heat to the colder walls of the cylinder. 
The ratio of loss is not great, however, and such as it is it 
represents the differential effect of the cooling of the walls 
and the heating by contact with and radiation from the hot 
piston. During explosion the gas is seen, both from the table 
and the curve, to gain heat rapidly until the point of greatest 
pressure and temperature is reached, and then the curve 
falls rapidly and the gas begins to show a loss of heat to the 
cooling walls. This loss has of course been going on during 
the explosion also, but the effect is masked by the far greater 

jtj 

quantities of heat then being liberated. If desired can 

be plotted on a time base. 

35. It is often found that during compression or expansion 
the gas will follow the law 

p. V n = constant =c, say 
so that rfp = rf(cF-") = _ weF _„_ 1 

dV dV 



■ — npV n x 



1 —_ n V 

yn + l y 



V.f v =-n p . 
Substitute this in equation' (2) 

and ^=yh{^±y p rl^ •• < 3 > 

A very simple expression which can often be used to obtain 
results speedily. If the gas lose in H during compression 
evidently (y—n) must be positive or y must be greater than 
n. During expansion, if the gas is losing heat, (y—n) must 
be negative or n be greater than y. 

This analysis was originally due to Professors Ayrton and 



THERMODYNAMICS 67 

Perry, and published by them in the Proceedings of the 
Physical Society in 1885. 

It has already been explained that the loss to the cooling 
jacket must go on during explosion as well as expansion, 
and that it should be possible to draw a curve of the time 
rate of cooling during explosion which should have an area 
corresponding to the quantity of heat actually known to be 
carried away by the cooling water. The difficulty, however, 
is to know how to allow at the same time for the heating or 
cooling effect on the gas of the hot piston. (The writer will 
show presently from an analysis of certain of Professor 
Burstall's results what kind of effect this is.) And an 
additional difficulty lies in the fact that the temperature 
of the gas is now known to vary very considerably in 
accordance with its proximity to the walls. 

Mr. Petavel has made some experiments on the loss of 
heat, e, per square cm. per degree Cent, by bright platinum 
in an atmosphere of C0 2 , at temperatures ranging from 
200° C. to 1,200° C, and at pressures ranging from 6 cm. to 
228 cm. and from his results Professor Perry has published, 
in his book, the following rule deduced by the writer — 

e = l-55 xlO- 8 p(l,OOO+0)+l-67 xl0- 6 fl 
where p is in pounds per square inch and the temperature 
is 0° C. This formula, or one of its type, has been applied 
to gas engine problems, but the results need not be quoted 
here. 

36. Effect of the variability of specific heat. Equations (2) 
and (3) were obtained from premises which assumed a con- 
stant specific heat, and it is desirable to see how they are 
affected when the known variability of specific heat is 
allowed for. Mr. Dugald Clerk has given (see the previous 
chapter) the following results obtained by Messrs. Mallard 
and Le Chatelier in 1883 — 

For C0 2 .. O„=0-1423+0-0000S3I 
H 2 .. C p =0-3116+0-000182 
N 2 .. C t ,=0-171 +0-0000215 
2 .. ^=0-150 +00000188 
In his Report to the Gas Engine Research Committee of the 
Institution of Mechanical Engineers Professor Burst all 



68 THE INTERNAL COMBUSTION ENGINE 

tabulates the somewhat different results published by 
Messrs. Mallard and Le Chatelier in 1887 — 

For C0 2 . . C ( ,=0-1477+0-000176 
H 2 .. C v =0-3211+0-000219 
N 2 .. C w =0-170 +0-0000872 
2 .. C„=0-1488+0-0000763 
From these later figures Professor Burstall has calculated 
for different mixtures of coal-gas and air the values of C v 
as shown in the curves reproduced. In 1902* the writer 



-0000/2 
■000011 
'000010 
•00009 

-18 
17 
■16 
■15 



Relation of Specific Heat at Constant 
to Ratio of Air to Gas 
K=A+ct° 



Volume 











0* 




























































A 












































Ratio 


Air t 


o Gas 































10 



12 



13 



14 



15 



■072 
■071 
070 
069, 




Relation 


of Gas Constant to Ratio of 
Air to Gas 

ft ~ ftp~f\y 




































































> 6 


/ 


6 


s 


I 


I 


1 1 


2 L 


3 I 


% IS 



Fig. 14. — Professor Burst-all's curves for specific heat constants for 
different ratios of air to coal-gas. 

analysed certain of Professor Burstall's results, using these 
curves for the variable specific heats, and certain of this 
work is reproduced here. The discrepancies between the 
different measurements of specific heat are so great that the 
only satisfactory procedure is to keep to one set of figures,, 
* Engineering, June 27, 1902. 



r R\ p ^ dV' 



THERMODYNAMICS 69 

and use them whenever numerial results are needed. When 
they are not, it is best to keep carefully to symbols, which 
can be better interpreted when reliable results are known. 
Let C p =a+s6 and C v =P+sO 

Nowi?=a — /3 and put-— =y . i.e. the value of y when# = 0°C. 
P 

Then SH=C V .$T+ —p.dV 

J 

(The introduction of J enables the calculation to be kept in 
heat units throughout.) 

therefore ^=C V .^ + ^ 

dV v dV^ J 

Now ^=JR 

T 

so that T=^,nd§ JR 

and ^ = ^L(p+7^L)+^r 

dV JR \^ dV> J 

This is the same equation as before, except that in this it is 
found convenient to keep in the constant J. At this point 
however C v must be expressed as /8+s0. 

therefore J dI L = l+f (p+V**-) + p 

dV a— 13 V^ dV J F 

ap+s 8 p+ (3V-^+sev^L 

d H 

and this is the new expression for — . 

dv 

If «s=0; i.e. if specific heats were constant, equation (I) 
would clearly at once become equation (2). 

As before, take the case where, as in compression and 
expansion, pV"=c very nearly. 



70 THE INTERNAL COMBUSTION ENGINE 

Then as before 

and substituting in equation (4) 

T dH y—n sO ( 

J—- = -^— - .p + \ p—np 

dv y — 1 a — p { 

T dH ( y — n n — 1 a ) . . 



dV \y — 1 a—8 

which becomes equal to equation (3) if 5=0. 

dH 
Equation (5) shows that -z— -is proportional to p when 6 

is constant, and that it is a linear function of when p is 
constant ; provided always that all changes are regulated 
by the law pV n — constant. 

J TJ 

37. Adiabatic law with variable specific heats. If-=- be 

dV 

zero, or, in other words, if the transformation be adiabatic, 

it follows from equation (4) that 

T7 dp . a+s6 

v w + j+re p=0 - 

If s =0 this would become 

17 d 'P 



■yp=0 



dV 

which on integration would give the familiar pV y = constant 
for the adiabatic equation. Here, however, s is not zero, 
and it is necessary to integrate 

dV^ f3+ s 6 P 
which gives 

p e i.7«i. e *? =C onstant 
where /3 1 and a t are the constants in the equations 
C p =a i -{-sT and C v = P ± +sT. 



THERMODYNAMICS 



71 



a ± =a — 274 s 



This means that 

and P t =/3— 274 s 

so that the adiabatic equation is 

^(3-274 S)> F (a-274 S)> e * =const ant, (6) 

e being 2-71828, the Naperian base. This gives a higher 
expansion curve than pF 7 ^ const ant, as is shown in the 
annexed diagram Fig. 15. Perhaps the best way of stating 
the law is 

P t log p-\-a t log F+s#=constant .. (7) 

It will have been realized from this that if the variability 

of specific heats be admitted a very considerable change 



15- 


200 

150- 

< 
o 
C 

<b 

\ % ioo- 

to «o 

to 

5 50- 














V z i 


Adiababics. 

1 










1 
/. Cons bant SpeclFi 


c Heab. 




10 - 


/-/ ^ 


\2 . Variab 


le Specific 


Heab. 








































5 - 










^T2 












r 














- 













4\6% 



20 
Clearance 



20 50 % 
Percenbage of Sbroke 



100 

-J 



Fig. 15. — Adiabatic Curves as calculated by Professor Burstall. 



must be made in the customary formula, and a good many 
papers and books based on the old figures must be looked 
upon rather as affording a mental training than as a guide 
to what occurs inside a gas engine cylinder. 



yo- 



38. Tn Prof. Burstall's engine tests, , was found to be a positive 



To" 



72 THE INTERNAL COMBUSTION ENGINE 

JIT 

quantity during expansion, so that had s been zero, then would 

have been positive, and with an increasing V this would mean that the 
gas was gaining heat during expansion. This could only occur when 
either combustion was still going on or the gas was deriving heat 
from the walls or piston. Naturally when faced with such a choice 
as this it has been usual to conclude that combustion continued until 
late in the stroke — a conclusion not in accord with experiments con- 
ducted in different but allied circumstances. Giving to s the value 
already indicated has the effect of making the second term in 
equation (5) larger than the first, for all temperatures considered, 
and so leads to the more reasonable conclusion that combustion ends 
at or near to the point of maximum temperature, and that for 
the remainder of the stroke the gas loses heat to the walls at a 
calculable rate. 

The writer has worked out some values for-^frfor the tests marked 

" D " in Prof. Burstall's investigation. Table I is composed of the 
leading data given in the Report, and Table II of results deduced 
therefrom. It is evident that the consistency of the values given in 
the last column of Table II — obtained as they are by the consideration 
of small differences — forms a very decided commentary on the 
extreme accuracy with which the experiments must have been 
conducted. It would not be easy indeed to devise a more rigorous 
test for experiments of this nature. 



Table I. 
1 kg. /cm. 2 = 0-97 atmosphere =14-2 lb. /in. 2 . 



D 

Tests 


Suction 
Pres- 
sure 

kg /cm 2 


Suc- 
tion 
Tem- 
pera- 
ture 
°C. 


Pres- 
sure at 
end of 
Com- 
pression 
kg/cm 2 


Tem- 
pera- 
ture at 
end of 
Com- 
pression 
°C. 


Maxi- 
mum 
Tem- 
pera- 
ture in 
Cycle 
6 C. 


Maxi- 
mum 
Pres- 
sure in 
Cycle 
kg /cm 2 


Ex- 
haust 
Tem- 
pera- 
ture 

°C. 


Ex- 
haust 
Pres- 
sure 

kg /cm 2 


Jacket. 
Water, 
Tem- 
pera- 
ture of 
Outlet 
°C. 


1 


1-00 


143 


8-66 


452 


1,437 


13-60 


862 


2-73 


65 


2 


100 


140 


8-92 


468 


1,509 


18-28 


822 


2-65 


62 


3 


100 


132 


8-82 


445 


1,442 


14-80 


872 


2-83 


62 


4 


100 


128 


8-70 


429 


1,454 


14-41 


887 


2-90 


64 


5 


1-00 


115 


8-70 


406 


1,372 


14-05 


842 


2-88 


66 


6 


100 


110 


8-85 


409 


1,245 


13-81 


777 


2-75 


60 


7 


100 


98 


8-66 


373 


1,145 


1218 


787 


2-86 


66 


8 


0-95 


84 


8-36 


359 


1,094 


11-85 


749 


2-72 


66 


9 


100 


84 


8-82 


360 


1,023 


12-60 


702 


2-73 


63 


10 


100 


69 


8-72 


327 


897 


12-00 


637 


2-66 


64 



THERMODYNAMICS 



73 



Table II. 
Columns 1-6 are taken from Prof. Burstall's report. 





n in 
PVn 

for 
Com- 
pression 










During Compression. 






D 


n in 
PVn 
for 
Expan- 
sion. 




1000 s 


E 


■;o 


yo — n 
To— 1 


dH 

J Jv at 


Tests. 


begin- 
ning of 

com- 
pression 


end of 
com- 
pression 


1 


1-345 


1-344 


01834 


0112 


00715 


1-390 


0116 


0-54 


-15-9 


2 


1 


364 


1-338 





1827 


0111 


00713 


1-390 


0067 


—017 


-25 1 


3 


1 


357 


1-324 





1811 


0-108 


00711 


1-392 


0089 


0-24 


-190 


4 


1 


349 


1-327 





1804 


0107 


00710 


1-393 


0112 


64 


-140 


5 


1 


349 


1-327 





1785 


0105 


00707 


1-395 


0116 


0-80 


-11-9 


6 


1 


359 


1-294 





1778 


0103 


00706 


1-397 


0096 


0-54 


-14-9 


7 


1 


345 


1-251 





1765 


0101 


00705 


1-400 


0138 


1-26 


- 5-8 


8 


1 


345 


1-245 





1756 


100 


00702 


1-400 


0138 


1-31 


- 4-5 


9 


1 


357 


1-230 





1756 


0100 


00702 


1-400 


0108 


0-92 


- 9-2 


10 


1 350 


1199 


01745 


0099 


00700 


1-401 


0127 


1-32 


- 4 2 



dH 
39. It is seen that ,y at the beginning of compression is positive 

(with one exception), and as V is decreasing during compression it 
follows that the gas is losing heat to the enclosing walls. At the 

dH 

end of compression, however, —jy- is negative without exception, 

showing that the gas is then gaining heat from the enclosure. 
Further, it is seen that this gain is greater as the maximum tempera- 
Table III. 



D 

Tests. 


Maximum 

Temperature 

in Cycle 

°C. 


Maximum 
Temperature 

in 
Compression 

°C. 


Suction 
Temperature 

°C. 


End of 
Compression 
dH 
J dV 


Beginning 

of 

Compression 

T dH 


2 


1,509 


468 


140 


-25 -1 


-017 


4 


1,454 


429 


128 


-140 


0-64 


3 


1,442 


445 


132 


-190 


0-24 


1 


1,437 


452 


143 


-15-9 


0-54 


5 


1,372 


406 


115 


-11 9 


0-80 


6 


1,245 


409 


110 


-14-9 


0-54 


7 


1,145 


373 


98 


- 5-8 


126 


8 


1,094 


359 


84 


- 4-5 


1 31 


9 


1,023 


360 


84 


- 9-2 


0-92 


10 


897 


327 


(ii) 


- 4 2 


1 32 



74 THE INTERNAL COMBUSTION ENGINE 





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d-uoui u 


pau 


ibS JS3}i 






*"*> ^ 




















. 


c e * 






















CO «0 «0 




c 


















*•*$ 






















u ft o 




tj 


















* J» ft 




J! 












ft 
'5 


ft 

o 






5 












«i 




- 






<5 












k 

o, 


X 

* 




gfci 
















o 


o 




4. V. <8 «ft 




.5 
















3 O «N 




c 












H» 


«] 




U « - C «9 




ft 












O 


V. 




fc 5 «8 * 
















ft 


"5 




*> » *• * 
















u± 


V 




5 * N 


















41 




U * -ft N 


















ft. 




0) u - n 


















t 

w 




» <: c u, 






































£ 


























£-**.*> 


















5 




*«!>,C 








































>< 




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s£|- 






















*J u c c 






















«»••.«, 






















4) *» t»\ «) 






















* ^ <U fc 






















K -^ O I 
























(liOtSS^jdulOQ 


JO 


puj) su/n/OA jo 


abueijo 








jiun 


uad sp 


uncd . 


-Uout 


Ul 


paui 


B6 ~)E9fi 







Fig. 16. 



THERMODYNAMICS 75 

ture of the cycle is greater. This is well seen in Fig. 16 in which 
the above results are shown plotted. 

cJT-f 
In fact, if, as in Table III, ^rrbe shown tabulated in a descending 

scale of maximum temperatures it is at once seen that the gain of 
heat by the gas at the end of compression increases with increase in 
each of these temperatures or in fact with increase in the mean 
temperature of the cycle. 

These results were of such an unexpected nature that it almost 
appeared that there must have been some slip made in the experiments 
themselves or in the analysis of them. The probable explanation was- 
however considered tc be, and later information has served to bear 
out its accuracy, "that the high temperature of the face of the piston 
has a markedly heating effect on the gas, especially when the piston 
is near the beginning of the stroke, and there is a relatively thin 
" slab " of gas between the hot piston face on one side and the 
cylinder end on the other. A tabular statement will serve best to 
bring this point out. 

(1) The gas is seen to lose heat slightly at the early part of com- 

pression. 

(2) This loss is greatest when the mean temperature of the cycle 

is least. 

(3) The gas gains heat rapidly at the latter part of the compression 

stroke. 

(4) This gain is greatest when the mean temperature of the cycle 

is highest. 

(5) The action of the enclosure on the gas must therefore be dual. 

(6) The gas loses heat to the jacketed surface which is cooler 

than the gas at the moment in contact with it. 

(7) The gas gains heat from the piston which during the compression 

stroke is always hotter than the gas. 

(8) This heating is seen from Table III to occur most when the 

maximum temperature of the cycle is high, and as the mean 
temperature of the cycle rises and falls roughly in accordance 
with the value of the maximum temperature, the maximum 
effect of the heating occurs also when the mean cyclic tem- 
perature is highest, which corresponds with what would 
reasonably have been anticipated on this hypothesis. The 
observed facts and the deductions based on them do, therefore,, 
harmonize with one another once the high temperature of 
the face of the piston is admitted. 
40. A later deduction may now be added. From (6) it follows 
that the skin temperature of part of the jacketed walls must fall 
during the cycle as low as 100° C. or even less. Now Dugald Clerk 
has found from certain measurements that the temperature of the 
hottest part 'of the skin rises sometimes to 400° C, suggesting a 
temperature difference between different parts of the cylinder walls 
of 300° C. Further that, as regards the piston, the skin temperature 
must commonly exceed 500° C, and possibly exceed it a good deal, 
even during the compression stroke, although there has been an 
interval of time corresponding to more than a whole revolution 
since the last explosion. It may therefore be doubted whether the 



76 THE INTERNAL COMBUSTION ENGINE 

temperature of the piston skin ever sinks below 500° C, and it may 
sometimes have an average value greatly exceeding this amount. 

The deduction that the temperature of the piston is very much 
higher than that of the walls occurred first to the writer on making 
these calculations more than six years ago when for the first time 
theory was made to take account of variability of specific heats, 
and it goes far to explain these temperature variations in the mass of 
the gas itself which were found by Prof. Burstall by means of his 
electric thermometer. A simple calculation based on the thickness 
of the walls, the conductivity of the material and the maximum 
power going to the cooling water (a plate of average iron will 
steadily transmit per square foot of area for a gradient of 10° C. per 
inch about 3 h.p.) will show that the average temperature gradient 
cannot be large, but what its instantaneous value may be it is more 
difficult to say ; this point will however be treated of further shortly. 
It is interesting in this connexion to work out what the actual 
loss in ft. -lb. per second would be according to Mr. Petavel's experi- 
mental results, remembering that it is, of course, impossible to get 
any exact idea of the kind of heating which occurs, owing to the 
large radiation from the piston masking the cooling effect of the walls 
and owing to the surfaces being of such different shapes and dis- 
positions. The writer has already quoted the formula. 

10 8 Xf=l-55p( 1000 + 0) + 1670. . .forC0 2 
where p is in lb. /in. 2 and is in degrees Cent. The formula is 
applicable between = 200° C. and 0=1,200° C. and up to three 
atmospheres pressure. Mr. Petavel has also given a formula for the 
•emissivity, e, from a hot platinum wire at high temperatures and 
pressures as follows : — 

e = ap n + bp m 8 
here 

e = emissivity in water gramme degree units per sq. cm. per 

sec. per deg. Cent. 
p = pressure in atmospheres. 

8 = temperature difference between the wire and the enclosure 
in deg. Cent. 
For C0 2 , between 8= 100 and 1,100 and between p= 10 and 35 
this equation becomes 

10 6 Xe=207p°- 82 +l-50p°- 3 M (8) 

The diameter of the wire used in establishing these results was 
1106 mm., so that the area per inch of wire was 0-884 sq. cm. 
From this equation it is possible to get some idea of the rate of loss 
of heat of a mass of gas for a temperature difference of, say, 1,000°C. 
between the gas and the enclosure. To do so it is, of course, 
necessary to extrapolate, since the temperature of the gas was 
found by Mr. Petavel to be only 6 per cent, of that of the wire 
reckoning from the temperature of the enclosure as a zero. Further, 
this means that to the first approximation the first term in equation 
' (8) may be neglected and that 

10 6 X €= 1-50 VpX 1,000 X — very nearly. 

6 
Xet p be 10 atmospheres, then 

6=53,500X10— 6 = 00535. 



THERMODYNAMICS 77 

This means that the heat loss per sec. per sq. cm. is 53-5 water- 
gramme -degree units. Now this is based on the consideration that 
as the gas was found to be 6 per cent, of the temperature of the wire, 
it may be taken that the loss of heat by a wire at a high tempera- 
ture would be approximately the same as the loss of heat by a mass 
of gas, with no wire, at 6 per cent, of that temperature. 

41. This does not appear an unreasonable hypothesis in view of 
the fact that it has been shown that pure radiation and conduction 
losses are of little importance when compared with the loss due to 
correction currents, currents which would certainly exist even if the 
wire were absent, so that in this particular example the loss per inch 
of " imaginary wire " 

= 53-5x 0-884x3-09= 146 ft. lb. /sec. = 0-265 h.p. 

To use such a result as this, interesting as it may be, for gas 
engine work would require a further knowledge of the laws governing 
the radiation of hot gases to cylindrical surfaces of different diameters, 
and at the best it would not be strictly fair to extend it to the con- 
sideration of a non-cylindrical surface, such as a piston surface, in 
contact with the gas. 

The above loss of 0-265 h.p. per inch length of cylinder would 
amount to about 2 h.p. for an average exposed length of 8 inches. 
The actual rate of loss during expansion is very much larger than 
this amount for the gas engine cylinder used by Prof. Burstall (viz. 
one 6 in. diam. by 12 in. stroke). 

It has often been remarked that the expansion curve for a gas 
or steam engine almost always follows a simple law of the type 

p V n = constant, 
and from this it has always been shown that 

^=(A + B6)p 

where A and B are constants. 

dH . dH , , . , . 1 dV 

Transforming -,y into —rr by multiplying by —■ -, - and then 

dV 
expressing -37 in terms of V (the revolutions per minute being 

constant) and then eliminating V by means of the equation: 

pV n = constant 

, . . . dH 

it is possible to obtain an equation giving -,- m term sof p and 6. 

dH 
Then since ~,. when divided by temperature and by area, gives the 

emissivity, it is possible to build up an equation showing, in terms 
of the emissivity, the differential heating effect due to the joint 
action of the cylinder walls and the piston. It is not possible, 
however, to follow such speculations to their end in a volume so 
small as this, but it is greatly to be recommended that students 
who are really interested in this fascinating subject should them- 
selves pursue the matter. It will give them, perhaps — and perhaps 
not — valuable information, but very certainly the tackling of the 
many difficulties which always occur in such investigations will 
piove the greatest assistance in familiarizing them with the subject 



78 THE INTERNAL COMBUSTION ENGINE 

and incidentally of convincing them how very little is known of what 
.goes on in a gas engine cylinder. 

42. Experiments on Measuring Temperatures during the 
Cycle of Operations in a Gas Engine. — Professor Burstali * 
was the first to do this. He used a platinum ther- 
mometer, and came to the conclusion f that it was im- 
possible, owing to the fusing of the fine platinum wire before 
a sufficient number of observations had been taken, to 
make such measurements ,with an engine working on full 
load. He had therefore to experiment on an engine running 
light and firing but once in each twelve revolutions. The 
principle upon which a platimum thermometer works is 
that since the electrical resistance increases with the tem- 
perature in accordance with a known law, to measure the 
resistance of the wire is to measure its temperature at 
the moment. Professors Callendar and Dalby J have 
recently made additional tests in this direction. These 
experimenters realized that they could not get a wire which 
would " stand up " to the temperature of explosion 
unless it was so thick that it must fail to follow the fluc- 
tuating temperatures of the gas with sufficient rapidity. 
They therefore decided so to arrange the apparatus that 
they could withdraw the fine thermometric wire from 
the action of the gases during explosion, and replace it 
for each suction and compression stroke. This was effected 
by fitting up the inlet valve as shown in Fig. 17. C is 
the admission valve casting, which is bolted on to the 
cylinder and projects inside the space provided for it. The 
thermometer was inserted through the spindle of the main 
admission valve marked A, which had been drilled out to 
receive it. In the figure the little "thermometer valve," 
as it may be called, is shown projecting beyond the main 
valve head into the cylinder. It closes with a little conical 
seating of its own as soon as the ignition point gets near. 
The thermometer leads enter through B, pass along the 
thermometer valve spindle until they arrive at the fine plati- 
num wire which is shown at P. The head of the ther- 
mometer valve is connected to its spindle by the two ribs 
* Phil. Mag., 1895. f Proc. I.M.E., 1901. % Royal Soc. Paper, 1907. 



THERMODYNAMICS 



79 



which are made as thin as possible so that the platinum 
wire is not screened more than can be helped from the 
action of the hot gases when the thermometer valve is pushed 
out into action. The opening and shutting of the thermo- 




Fig. 17. — Combined Admission and Thermometer Valve (Callendar). 

meter valve at the proper times is effected by suitable me- 
chanism. The thickness of the platinum wire was yxroo" 
of an inch. At about 130 r.p.m. the lag of the thermometer 
was not more than 10° of crank angle with a temperature 
fluctuation of nearly £00 in half a revolution. This would 
correspond to a time lag of -£$$ x ^ =0'06 sec. which was 
quite good enough for measuring anything so relatively 
steady as the suction temperature. As a result of such 
measurements it was found that the suction temperature 
varied with the conditions of running from about 95° C. at 
light load to about 125° C. at full load, the air temperature 
being about 20° C. and the jacket temperature 27° C. The 
following are details of two tests — 



R.P.M 

Ratio air /gas 

Atmospheric temperature . 
Jacket temperature . 
Temperature of thermometer 

360° crank angle . 
Ditto at 26° crank angle 
Corresponding pressure at ditto 
Molecular contraction on combustion 



alue at 



Test I. 



Test II. 



130 

71 

20° C. 

27° C. 

122° C. 
111°C. 



114 

5-8 
21° C. 

27° C. 



130° C. 



lb. 



18-5 lb. /in. 2 17-8 

4-3 per cent. 51 per cent 



m. 



80 THE INTERNAL COMBUSTION ENGINE 

It was noted that by a curious coincidence the indicator 
cards from these two trials showed a practically identical 
expansion curve, not varying by more than 1 lb. /in. 2 at any 
point. The temperatures during expansion were however 
far greater when using the richer mixture and the heat losses 
to the walls correspondingly greater, so that although much 
more gas is used in one case than in the other, no more h.p. 
is obtained, the excess heat units going to waste. 

These experiments show also that the common practice 
of assuming the suction temperature to be 100° C. irrespec- 
tive of load is an inexact one. When, as in the above ex- 
periments, the suction pressure is accurately measured, say 
within +1° C.j it is possible to calculate accurately the 
temperatures throughout the cycle. 

43. It has been shown that the adiabatic equation with 

variable specific heats — see equation (7) — is 

/3 1 log p+a 1 log V-{-sO= constant. 

Where C v =fi 1 -\-sT and C p =a 1 -\-sT, this could equally well 

be written 

f3 ± log p-\-a ± log V -\-s T= constant. 

vV 
Now T=^~ so that the equation can be written in terms 
R 

of the variables p and V only as 

/3 t log p+a x log V+—-pV= constant 

Now put — - =y 1 ; i.e. the value of y when T = 0, 
Pi 

therefore log p-\-y 1 log V-\— — pF=constant .. (9) 

Rp l 

It will be seen that this differs from the older form pV y = 
constant (which can be written of course as log p-\-y log 
F=constant) in the addition of the third term on the left of 
the equation. This term of course drops out when s=0. 

This is all based on a linear relation between specific heat 
and temperature, but as shown in Fig. 18 Dugald Clerk's 
measurements of specific heat as quoted in the preceding 
chapter, are nearer a parabolic relation. If however 
one only of the observations, that at the highest tern- 



THERMODYNAMICS 



81 



perature, on which this curve is based, be omitted a very 
fair straight line will lie among the rest, and this line can 
be sufficiently closely indicated by the equation 

C- 0194+0051 — — (10) 

1,000 } 

this is for a 1/9 mixture, and Burstall for such a mixture gives 

£ w = 0-178+0-105— °— (11) 

1,000 v ' 

which shows a rate of increase of about double Dugald 

Clerk's. Which therefore is one to choose ? 



30- 








P0~ 




BR^ 


BllI^ 




<£. 








•+0 








G 5 








>*: t- 








CD 






10- 


£ to 

<U 1 






0- 




1 


— 1 1 



500° C 



100 0°C 



1500° C 



Fig. 18. — The heavy line shows Dugald Clerk's apparent specific heat 
curve based on the figures given on p. 55. The horizontal line 
corresponds to a constant specific heat and the shaded area to the 
difference between the two. 

For 0=400° C, they both give nearly the same result, i.e. 

the former gives C v = 0214, and the latter C v = 0-220, but 

at 1,600° C. the former equation gives 0-275, whilst Burstall 

gives the much higher figure of 346. It is difficult to 

choose between these two as the systems of experiment 

upon which both are based are open to criticism to about 

the same degree. Further experiments are greatly needed.* 

44, Effect of variable specific heats on Calculation of Efficiency. It 
is now important to see how the calculation of efficiency is affected 
by the adoption of a linear equation for the specific heat. The 
most important cycle is the " Constant volume " one, and in that 
it will be remembered (see par. 18) that it was found that 

( T— r t )— (izv- t ) 

* See the appendix to this chapter. 



82 THE INTERNAL COMBUSTION ENGINE 

For convenience of reference Fig. 6 is reproduced in Fig. 18a. 



25 



10 

Id 

tn 20 

HI 

X 

a 

1 - 



< 

z 

" 10 

ill 
a 

D 
</) 

a 

a 















































Jf« 












































































P/ 


























V 



















































2500 



•5 
VOLUME 
-- Vo — 




•I -2 

ENTROPY 



Fig. 18a. 

With a variable specific heat the calculation of the thermal 
efficiency is far more complex. Thus heat taken in 



and heat rejected 



(T 3 —T )[p 1 + 



' 2 
T + T 3 



so that 



(T.—T^ + s 



T,+T. 2 



{T,—T )[$ 1 + s 



Ta + Ti 



Vo 



(To—T^ifr + s 



Ti + Tf 



where r) is the new efficiency. 

It is now necessary to reduce this expression in some way. It 
pV 
still holds good that -~r = R, and from equation (6) any adiabatic 

change must follow the law 



pV yi e ft = constant. 
Combining these two it follows that 

P3V3 

and p 2 V 2 yu 1 =p 3 F 3 ne0i 2 

T 2 /F 3 \7i-i s 



P2V2 

T'2 

s 



so that 



vj 



ejSi 



(T3—T0) 



and calHng ~ , r the ratio of compression as before 



THERMODYNAMICS 



83 



(T3—T2) 



and 



T 2 1 A 



— - = ryi x e jbi 



Substitute for T 3 and T in the equation for rj and 



>-°>][/3 1 + ^] 



5 — r * s ) ( t -\-Tr,\ — 1 

iT r -T t )(^ + ± { T i +T 2 )) J 



t/o^ 1 



r ) 



Now the numerator in the square brackets must be expanded — 
LI T 2 .e r ti {T 2- T z ] — t ~ik ( T i~ T o) - \ ft. + -| ( 2V 1 — n-e^ (r 2 -T 3 ) 



+ 2 7 i_ yl . e — (n— r ) 



] 



Now expand the exponential terms to the first two terms. (Students 
would do well to try the exercise of expanding to three terms. ) The 
numerator now becomes 

[t 2 (i + ±- { t 2 — T 3 ))— T 1 (l + ^(T 1 -T '))}{/3 1 + -|ri-n 
[ ra ( 1+ ^^,,) + r 1 (l+^^ 0) )]J 

= {^2— T 1 + ~^{T 2 *— T 3 T 2 ~T^ + T 1 T 0) }{^+ S 2 ri-yi[T 2 + T 1 + ^- 

(T 2 *—T 2 T 3 + T 1 *—T i T )']} 

Now the left half of this expression can be written to a sufficient 
degree of approximation 

s ) 

T 2 —T 1 + —-(T 2 2 —ToJri-yi—Ti> + T i -ri-yi)\ 
Pi 

= [r 2 — T x + ~[r 2 2 (l— ri-n)— ^-(1— ri-n)]} 

= T 2 —T i + 4-(l— ri-yi)(T 2 *— T^) 

Pi 

= (T 2 —T 1 ){l+ ^(1— r 1 -n)(To+2\)| 
Therefore the whole square bracket terra can be written 



84 THE INTERNAL COMBUSTION ENGINE 

'(T 2 — 2\)-[ 1+^(1— ri-yi)(T 2 + T 1 )}{l + ^ri-yi[T 2 + T 1 + 



<^'(-i-^- 2 ) 



Pi 

Then neglecting terms involving ( — — J this expression can be re- 
duced as follows : — 
{ 1 + ^(1— r^i){T 2 + T 1 )f\l + ^ri^[r 2 + T 1 + -^(T 2 *--T 2 T 3 

+ T t — zyro)j, [i— ^ 2 

= 1 + ^(1— ri-n) (T 2 + T 1 ) + 2^ri-yi(jr 2 + T 1 )— ~ ^±^i 

= 1 + £- { T 2 + T 1 —T 2 r^-yi—T^-yx + \T 2 r^~yi + \T x r^-yi— 



\T 2 —\T, 



= 1 + j Ut 2 + \T X — |T 2 ri-yi— PV-i-nJ 

= 1 + 2 ^{ T 2 (l— ri-n) + 2^(1— ri-yi)} 

= l + ^-(T 2 + T 1 )(l— ri-yi) 

Therefore ^= 1— (^"'[l + 3^ + ^(l— ri-n)] 



And since = 1 — r 1 — yi = 



it follows that 77 = n—r 1 ~yi^-r ] (T 2 + T t ) 



= ^{ 1 -27 1 (T2 + Tl)rl ~ n } 

or 7/o = ^{ \—^{T 2 + T x ) (l—r,) 

which may be written 

^{l-d-^^} (12) 

45. Equation (12) shows how the variability of specific 
heat enters into efficiency calculations. It will be 
noticed that T t the compression temperature enters 
into this equation, but since this temperature is so 



THERMODYNAMICS 85 

largely affected by the compression ratio it is better to 
replace it by the suction temperature which is nearly con- 
stant. We can do this by reflecting that as in Equation 

(12), T x is multiplied by -^, an approximation to T Y may 

Pi 
be substituted for it as follows : — 

— r n— i _|_ terms in -— . 
So that for sufficient accuracy for the present purpose 

T„ i-n 

therefore ^=^i_(i_, ; )_i_(r 2 + Jjl)} 

=^-w^ J * +T 4 •• •• (13) 

This is the important equation we are seeking. 

46. In working out examples by means of this equation 
and getting comparative results it will suffice in most 
cases to give to T some probable average value. When 
T is actually known, the real value may be used, but for 
working out a series of results it is best to take a round 
figure for T . A good average figure is 400° abs. , which corre- 
sponds to 127° C. The new efficiency equation then becomes 

*o=^l-^(l^Ts+400)J •• •• ( 14 ) 

To get numerical results from this equation it is neces- 
sary to insert the specific heat constants. For the purposes 
of illustration it will be interesting to take the figures 
already given for an average working mixture as based on 
the work of Mr. Dugald Clerk, viz. — 

- 0-194+0 051- 

1,000 

Mr. Dugald Clerk gives the weight per cubic foot at 0° C. and 
760 mm. of this substance as 0-07833 lb. This enables C p to 



86 THE INTERNAL COMBUSTION ENGINE 

be calculated, as the density relative to air will be 0-07833 -^ 

0-0807=0-97. 

So that C n —C=— =0-071 



v 



J 



It follows that O p = 0-265+0-051 



1,000 

At the absolute zero of temperature C v would on this law 

become 0-180 and C p , 0-251. So that 

251 , 
y ± = — =1-40. 
11 180 

If for example r = 7 

/ 1 \ °' 40 
1 = 1— I— J =0-54 

Substituting in our equation (14) above we have 

* =0:54<! l—^ 51 . _ ] _(o-46To+400) 
' 0-360 1,000 ; 

= 0-54(l^°^VH0O 



fp-943— T * ) 
V 15.400/ 



7,060 ) 

So that >/, =0-54^0.9.43— ) (15) 

V 15,400/ V ; 

T 

or «=0-51— *— (16) 

28,400 V } 

47. Both these equations are interesting. The former 

shows the factor, viz. 

^_\ 

15,400- 

by which the >/ efficiency equation must be multiplied in 
order to get the true value. It will now be of interest to 
work out this result for the cases in which ^2=273-1-1,600 
and 273+1,000. In the former case 

fl n =0-51— -Mi 3 - =0-51— 07 

28,400 

=0-44, instead of 0*54, 
and in the latter case 

3 n =0-51— 1,2/3 =0-51—0-05 

28,400 

=0-46 instead of 0*54. 



THERMODYNAMICS 87 

It has been alleged that owing to increase of specific heat the 
value of j] tends to a limit as the value of r increases. An 
examination of equation (12) 



-|HHt^) 



ft 

sheds a good deal of light on this suggestion, and shows it 
to be untrue. The equation can be used to do this as 
follows — 

The total heat given up on explosion being called H it 
follows that 

H=(T 2 —T x )(p x +s Tl + T2 } per lb. of mixture. 

H _ 1 s T,+T 2 

Substitute for this in the equation for r lQ and we have 

'1 ft(r 2 -7\)f 

Now T 2 — T 1 changes little even for a big change in r , so 
that it may almost be treated as constant in an expression 
which is multiplied by the factor (1 — y), which becomes 
very small as the compression is increased. 
Writing this nearly constant factor 

H 



PATr-TS, 



asP 



we have 



, =,{2-,-P(l-,)} 
=v{2— P+ri(P— 1)} 
Or r, Q =(2—P)ri+r,*(P—l). 

48. Now, does >7 tend to a limit as >/ is increased up to 
unity ? Differentiate and equate to zero, 

then 2— P+2>/(P— 1) = 

p 2 

Therefore when rj = the value of w n is a maximum. 

2(P— 1) 



88 THE INTERNAL COMBUSTION ENGINE 

But 

H s T 1+ T 2 

r-friTz—TJ T ft 2 

an expression but little greater than unity in almost all 
cases. 

P— 2 

Therefore is negative and there is no limit to 

2(P— 1) 6 

which *i tends as jj increases from zero upwards. The 
equation 

*o=(2— P)V+(P— lh 2 •• •• (17) 

happens incidentally to be one which can be used to deter- 
mine values of >; for different values of *y. The value of P or 

s T -\-T- 
1 +-~- • — ^ — 2 would of course require also to be known. 

-p * s 0051 1 1 

Putting — — = • or 

ft 0-180 1,000 3,530 

, T ± +T 2 



= (-¥ 



,060 

Now if (T ± +T 2 ) were, say, as high as 2,118 deg., then P = 1-3 
and % = (2— l'3)»;+(l-3— 1)>? 2 

=0-7^+0-3^ 

=0'3ri(2'3+Ti). 
If tj were 05, 07 or 09 ; ij would be 042, 0-63 and 086. 
Not too much must be built on these actual figures as they 
assume that (T x -\-T 2 ) is a constant and equal to 2,118 deg., 
which corresponds to a temperature half-way up the 
explosion curve of 

2 ' 118 — 273 =1.059—273 =786° C. 
2 

It is manifest that to keep this temperature fixed at 786° C. 
whilst the compression ratio is constantly changing repre- 
sents an artificial state of affairs, but equation (17) can of 
course be used, when this temperature is known, in any 
individual case. Students are recommended to calculate 
out specific cases for themselves and draw a curve showing 
the result. For calculating out theoretic efficiencies for 
the constant volume cycle the author recommends the use 
of equation (14). When values of specific heats are better 



THERMODYNAMICS 89 

known the calculations can be revised by inserting the more 
accurate constants. 

49. Mr. Dugald Clerk in his 1907 paper before the Institu- 
tion of Civil Engineers calculated how the efficiency would 
be affected by the adoption of the values for the specific 
heats found in his experiments. This he did by a graphical 
method and a process of successive approximations. The 
following table shows the results of his calculations. 





Ideal Efficiencies. 


1 


If Maximum Tem- 


If Maximum Tem- 






perature of Cycle 


perature of Cycle 


On Air Standard. 




1,600° C. 


1,000° C. 




1 

2 


0195 


0-200 


0-242 


1 
3 


0-286 


0-293 


0-356 


i 


0-354 


0-356 


0-426 


1 


0-384 


0-394 


0-475 


7 


0-439 


0-443 


0-541 



This table shows the very marked way in which the older 
expression for the efficiency is affected, when allowance is 
made for the variability of specific heats. It will be ob- 
served that the change is in the same direction, and of almost 
the same amount as that indicated by the preceding calcu- 
lation. The maximum temperatures, viz. 1,600° C. and 
1,000° C. are of course very low ones as they are the tempera- 
tures which occur at the highest point of the ideal cycle and 
not of the real one. The actual maximum temperature 
corresponding to Dugald Clerk's maximum of, say, 1,600° C. 
would of course be many hundred degrees less. 

The procedure which the author suggests, viz. to calculate 
by formula (14) is certainly a much quicker and more general 
method than any graphical treatment could possibly be. 
Students are advised to draw up a complete table from this 
formula, and to consider why in certain cases the values 
found for the efficiency differ from those given by Dugald 
Clerk. In Fig. 19 is shown the result of certain experiments 
made by Professor Hopkinson and reported to the Institution 
of Mechanical Engineers in his 1908 paper. The uppermost 



90 THE INTERNAL COMBUSTION ENGINE 

dotted curve is the old " Air Standard " which for the com- 
pression selected (viz. r=6-37) comes out at 52-2 per cent. 
Under that is a line which was calculated by Hopkinson on 
the basis of a variable specific heat (using the figures of Hol- 
born and Austin, and Langen). Below that again is the fine 



60- 



40- 



20- 



"Air Standard " Efficiency ( 52-2 % ) 



Hopkinson 's Curve of Ideal 
limiting Efficiencies . 



Actual thermal Efficiencies 
as measured by Hopkinson . 



Percentage of Coal gas in Cylinder Contents 

t — 



10 



Fig. 19. — Hopkinson's measurements of actual thermal efficiency for 
mixtures containing from 8 to 12 per cent, of coal gas, compared with 
his calculated ideal limiting efficiency curve. Note the falling off in 
efficiency owing to increase of specific heat, as the mixture increases 
in calorific value. 



of efficiencies as actually found. The second fine was not 
calculated by any formula, but by an approximate, and 
laborious, graphical method. Taking the maximum tem- 
peratures as estimated by Hopkinson, the author's for- 
mula (14) gives an efficiency of 40 -2 per cent, where Hopkin- 
son's line gives 39-4 per cent., and 41 2 against his 42 4. The 
difference between the Professor's line and the author's 
formula is due to the calculations being founded on slightly 
different figures for the specific heat, and to the fact that 
the graphical method is only an approximation. 

50. Exercise. What change would be effected in the 
thermal efficiency of an engine if working fluid were changed 
for one having a larger specific heat ? This is an important 
problem, as it not only concerns change of working fluid, 
but also whether it is well from the efficiency point of view 
to work high up the temperature scale or not. 



THERMODYNAMICS 91 

The thermal efficiency of an engine depends on many 
factors, but to a first approximation it may be said to be at 
least proportional, for any given compression, to the efficiency 
as obtained from the " Air Standard " formula in which 

where ^=ratio of specific heats. 
Now C P —C V =R and y —l=— 



hA 



so that v = l—(—\c v 

To find change of n with respect to C v , differentiate after 
transforming the above equation slightly 

log (l—rj) =_log— = — —-log r. 

C v r C v 

therefore .-= — . log r. 

l-i dC v C v * 8 

d n R(l-rj) . 

log r. 



tin R (\ \* . 

dc: = --o?'\Tr- logr - 



Therefore with increase of specific heat the efficiency falls. 
This could also be written 

This gives the fractional change in efficiency for a given 
fractional change in specific heat. 

If for example 7 = 1.40 and r = 10, then for a 1 per cent, 
increase in C v the corresponding fractional decrease in 
efficiency would be 

-1 1(1-4— 1)— Wiol 

100 V } n ^ ) 



92 THE IXTERXAL COMBUSTION EXGIXE 

Xow when r=10 >;=0-60 

i i i • C - <r 0-40 _ /| • 1 

and the above expression^ 0-40 x x2-3o - x — 

1 L 060 J 100 

= 61 per cent. 

So that in this ease the efficiency falls by rather more than 
^per cent, when the specific heat rises by 1 per cent. 

Exercise. If the maximum temperature on the ideal cycle 
corresponding to any given real one be not known, show 
how formula (14) can still be employed to find the limiting 
efficiency for various compression ratios, provided that the 
composition of the charge and its calorific value be known. 

To begin with, the heat given out on explosion =( calorific- 
value of the gas in C.h.u. per pound) x (weight in pounds of 
the gaseous mixture). In the ideal cycle this will be used 
in heating the gas from T± to T 2 so that heat absorbed = 
(To — T t ) x(mean specific heat of the gas between T x and T 2 ) 
x (weight of gaseous mixture). The weight of the gaseous 
mixture occurs in both expressions and may be cancelled 
out when they are equilibrated. 

So that {T 2 — T\) x (mean specific heat)= calorific value. 
Call the calorific value of the gaseous mixture per pound K. 

then (To—TJS. J r .s r 2± T A =K 



T 
Also Ti= — — to a first approximation and in this way T- 
1 — )] 

can be expressed hi terms of K and T and substitution for T 2 
made in equation (14). This will enable the value of the 
limiting efficiency to be calculated for any given richness of 
gas and ratio of compression. 

51. Later Measurements of Specific Heat. Reference has 
already been made to the uncertainties of the constants in 
the linear equations for the specific heats of the gases used 
in the Internal Combustion Engine. The results which are 
expressed in symbols are of course true for any values of the 
constants, provided only that a linear law fits the facts. 
Measurements of specific heat made by Messrs. Mallard and 
Le Chatelier. have already been quoted in par. 36. 



THERMODYNAMICS 



93 



Later measurements have been made by Holborn, Austin 
Langen, Dugald Clerk and Hopkinson, and the author made 
examination of them to see whether there was sufficient 
agreement between these measurements which would enable 
the old Mallard and Le Chatelier figures to be improved upon. 
The result has been negative. Although it seems possible 
that the old experiments gave a somewhat too sharp rise 
of specific heat with increasing temperature, yet the dis- 
crepancies between the later measurements are too numerous 
to enable a -decided statement to be made. For the present 
it suffices to use the older figures.* 

It is perhaps worth while recording that Messrs. Holborn 
and Henning f as a result of measurement made up to 
1,440° C. found that 

The mean value of C^ between 0° C. and 0° C. was 
for N 2 :— 0-2350+0-000019 6 (a straight line) 
and C0 2 :— 0-2010+00000742 0—0-000,000,018 2 (a 
slightly curved line). 

Calculated out these become — 





6°C. 


(■ 


o 




N 2 


C0 2 


200 . . 




•243 


•229 


400 . . 




•250 


•250 


600 . . 




•258 


•271 


800 . . 




•265 


•285 


1.000 . . 




•273 
•281 


•295 


1,200 . . 




•301 


1,400 . . 




■288 


•303 



The interested student can compare these with the results 
previously given and with those that follow. 

52. It is now becoming a common practice to give the 
specific heat in a new form. Instead of defining it as the 
quantity of heat in thermal units required to raise unit weight 
of gas through 1° Centigrade, it is measured as the amount 
of heat in ft. -lb. required to raise 1 cubic foot of the gas 
(measured at normal temperature and pressure — N.T.P. — i.e. 
0°C. and 760 mm.), through 1° Centigrade. This is rather 
* See Appendix in this chapter. f Engineering, January 3, 1908. 



94 THE INTERNAL COMBUSTION ENGINE 

better than the old way. as it is easier to measure volumes 
of gases than their weights, and the 4i ft.-lb. " form is 
obviously c onvenient . * 

Take for instance nitrogen which at a given temperature 
has a specific heat at constant volume of 250. In other 
words, 1 lb. of nitrogen will require 0250 C.h.u. to raise 
its temperature through 1° Centigrade. Now convert this 
into the other way of reckoning. 

1 cu. ft. nitrogen at X.T.P. weighs 0-078 lb., so that 1 cu. 
ft. will require 0-250 x 0-078 x 1,400 ft.-lb. to raise it through 
1° Centigrade, or 272 ft.-lb. Mr. Dugald Clerk's recent 
experimental results have already been recorded in this 
notation, and can be compared with this figure. 

As it has been found experimentally that the product of 
specific heat (at constant volume) by molecular weight is a 
constant, or nearly so, for all ordinary gases it follows that 
those gases will absorb about the same amount of heat per 
cubic foot when raised through 1° Centigrade. Thus if the 
value of the specific heat be given as C.h.u. per pound of gas, 
it is necessary to divide this figure by the number of cubic 
feet that go to 1 lb. of the gas in order to get the number of 
C.h.u. absorbed per cubic foot. This divisor will be inversely 
proportional to the density, and — since the density is pro- 
portional to the molecular weight — to the molecular weight 
also. To divide by something inversely proportional to the 
molecular weight is equivalent to multiplying by the mole- 
cular weight or by something proportional thereto. There- 
fore to get the old measurement (C.h.u. per pound) into the 
new one of ft.-lb. per cubic foot it is necessary to multiply 
by the molecular weight, and then by a constant. From this 
it will be seen that, to the extent that the above law is true, 
the value of the specific heat expressed in this way will be 
the same for all gases. This is obviously convenient. 

53. It is convenient to remember that to change from the 
old notation to the new, all that is necessary is to 

* The Gaseous Explosions Committee of the British Association 
suggest that the specific heat of a gas when expressed in calories 
per gramme-molecule should be called the ' ; volumetric heat." 
" Volumetric heat " = 3-96 x specific heat in ft.-lb. per cu. ft. 



THERMODYNAMICS 



95 



multiply by 078 and 1,400 in the case of N 2 
„ * „ 0244 „ 1,400 „ „ „ C0 2 
and „ „ 0089 „ 1,400 „ „ „ 2 

Mr. Bairstow * has deduced from a study of various experi- 
mental results the table given on p. 96, which he considers 
50,000 



40,000 



© Clerk 

O Lang en 

9 Ho/born and Austin 

x Mallard and Le Chatelier 




/ 



1,000 



2000' 



Temperature , Centigrade 

Fig. 20. — Hopkinson's Curve showing total heat (measured in mechanical 
units) necessary to heat gaseous mixture from 100°C. to the tempera- 
tures shown. 

to show more accurately than has been done before the 
specific heat of a mixture produced by the explosion of 
nine parts of air to one of coal-gas. 

* Engineering, June 7, 1907. 



96 THE INTERNAL COMBUSTION ENGINE 



Table I. — The Quantity of Mixture corresponding to this 
Table is 1 Cubic Foot Measured at Deg. Cent, and 
30 In. of Mercury. 



Temperature 


Absolute 


Mean Specific 


Specific 
Heat. 


Centigrade. 
0. 


Temperature. 
t. 


Heat from 
0° Cent. 






Ft. -lb. 


Ft. -lb. 





273 


— 


19-6 


100 


373 


19-6 


19-7 


200 


473 


19-7 


19-9 


300 


573 


19-9 


201 


400 


673 


201 


20-6 


500 


773 


202 


21-2 


600 


873 


20-4 


22 


700 


973 


20-6 


22-9 


800 


1,073 


210 


24 


900 


1,173 


21-4 


25-2 


1,000 


1,273 


21-9 


26-4 


1,100 


1,373 


223 


27-7 


1,200 


1,473 


22-8 


29-0 


1,300 


1,573 


23-3 


30-3 


1,400 


1,673 


23-9 


31-6 


1,500 


1,773 


24-5 


330 


1,600 


1,873 


251 


34-4 


1,700 


1,973 


25-6 


35-8 


1-800 


2,073 


26-2 


371 


1,900 


2,173 


26-8 


38-5 


2,000 


2,273 


27-5 


391 



And based on these figures he calculates by a method of 
dead reckoning the following table of efficiencies. 









Corrected 




Efficiency. 


Corrected 


Efficiency 




Air Standard. 


Efficiency 


Relative to Air 
Standard. 


4 


0-426 


0-335 


0-787 


5 


0-475 


0-384 


0-809 


6 


0-512 


0-417 


0-815 


7 


0-541 


0-445 


0-823 


8 


0-565 


0-470 


0-832 


9 


0-585 


0-490 


0-837 


10 


0-602 


0-508 


0-845 



Professor Hopkinson has endeavoured to show that after 
all the differences between the results of various workers 



THERMODYNAMICS 97 

are not great. This he does in the curve shown in Fig. 20, in 
which he has integrated the specific heat equations so that 
the curve shows the total heat in the gas (the same mixture 
as that described above) at any moment. This integration 
of course masks the differences. At certain temperatures 
good agreement could be obtained between results which 
showed great difference in the value of s but had the value 
of /3 corrected to balance this. The shaded area in the dia- 
gram is the debatable field in which so many battles have 
in the past been fought. 

54. Dr. F. Haber * has given so interesting a summary 
of the specific heat measurements for C0 2 that it is worth 
while quoting it here. In this summary C means the product 
of the specific heat at constant volume, and the molecular 
weight (which for C0 2 is 44). 

(a) 0=433 (JLV-367 acC ording to Mallard and Le 

Chatelier (explosion method). Basis : Explosive pressure 
in gas explosions at 2,000° and Eegnault's numbers. 

(b) 0=6-5+2-6 xlO -3 according to Langen. Basis: 
explosive pressures at 1,300°, 1,500° and 1,700°. The calori- 
metric determinations of Holborn and Austin between 0° 
and 800° agree with this expression. 

(c) (7=7-771 +000189 0, according to Schreber. Basis : 
Langen' s above mentioned experiments. 

(d) O = 65 + 000387T according to Mallard and Le 
Chatelier. Basis : experiments with the Crusher mano- 
meter. 

(e) It may be added that Le Chatelier later concluded 
that the specific heats of all gases under constant pressure 
converged towards 6 5 at the absolute zero. The values 
which he considered as the most probable Avere — 

Permanent gases 65+00006 T 
Water-vapour 65+00029 T 
CO, 6-5+00037 T. 



* Thermodynamics of Technical Gas Reactions . (Longmans. 
Green & Co.) 

H 



98 THE INTERNAL COMBUSTION ENGINE 

Dr. Haber gives a similar summary for water vapour, but 
reference should be made to his very valuable book for this 
and other important summaries of the same kind. 

Holborn and Austin have also given the following in- 
teresting comparative table of values of C v at various tem- 
peratures between 0° C. and 800° C. as found experimentally 
by various observers. 



Re°;nault. Wiedemann. 



Mallard and 
Le Chatelier. 



Lansjen. 



Holborn and 
Austin. 






0-1870 


01952 


0-1880 


0-1980 


0-2028 


100 


0-2145 


0-2169 


0-2140 


0-2100 


0-2161 


200 


0-2396 


0-2387 


0-2390 


0-2220 


0-2285 


400 


— 


— 


0-2840 


0-2450 


0-2502 


600 


— ■ 


— 


03230 


0-2690 


0-2678 


800 


— 


— 


03550 


0-2920 


0-2815 



55. Exercise. — If Van der Waal's equation for the relation 
between p, v and T be true, find the form taken by the 
equation for the specific heat at constant volume. 

Van der Waal's equation was 

(p-^)(V-n)=RT 

where m, n and R are constants. 
This can be written in the form 



V 



RT 



m 



which is of the type 

p=bT+a 

where b and a are functions of V only. 

Now dE=dH—p.dV 

and dH can be written, generally, as C v .dT+l.dV where I 

is any quantity. 

In the case of a gas in which the law vV —RT is obeyed, 
l—p, but in the more general case now being considered we 
cannot assume this identity. 
Therefore . dE=C v .dT+(l—p).dV. 



THERMODYNAMICS 99 

By the 1st law of Thermodynamics this must be a complete 
differential, so that 



/dC,\ /dl\ f*P\ 
\dvJ T \dTJv \dT/v 

Again d j>J^=^.dT+±-dV 

and by the 2nd law we must have 

I 

'2 



(1) 



(«£*\ =(^ 



1 (dC v \ 1 (dl\ I 

T\dvK~~T'^K T< 

= (*.) —1 .. (2) 

\dV J t \&Tly T v ; 

Combine equations (1) and (2), 

therefore (*L) =±ovT.( d l-) =1. 

\dTJv T \dTJy 

Differentiate and 

/ d*p \ = (dl\ _( d P\ 
\dT*)v \dT> v \dTJy 
or by (1) 

( dC v \ =T ( d 2 p \ 
\dV' T \dT*J y 

Integrate and we have 

C v — (a function of temperature only)-\-T • I ( — — ) • dV 

But by Van der Waal's equation we have 

p=bT4-a or ( — — ) = zero. 

Therefore C v —a> function of the temperature only. 

56. Flow of Heat through Cylinder Walls. A very pe- 
culiar and interesting problem is presented in the mechanism 
of the flow of heat through the walls of the cylinder to 
the cooling water. Before going into the question in any 
mathematical detail it is well to consider what are the 
dimensions of the various units concerned. The tempera- 
ture inside the cylinder follows roughly a sine curve dis- 
tribution in respect of time, but for simplicity assume that 



100 THE INTERNAL COMBUSTION ENGINE 

the inner skin of the wall is raised suddenly to, say, 400° C. 
and kept at that temperature for a time equal to one stroke 
— say -q^-q minute, i.e. 300 r.p.m. — and then lowered to a 
temperature of, say, 350° C, a far more violent oscillation 
than would be likely to occur. So for 010 sec. a con- 
siderable temperature difference exists between the two 
faces of the cylinder walls. How much is this temperature 
difference ? For argument's sake put it at 400 — 300 = 
100° C. Then for a 1 in. thick wall the gradient would be 
100° per inch, which would give (see par. 40) a heat flow of 

about 3 x — h.p. per square foot of surface, and this equals 

30 h.p. during the first of the four strokes of a four-cycle 

engine. Now a 10 in. diameter cylinder with an 18 in. 

stroke will yield about 30 h.p. at normal speed and its 

cooling surface area would be 

18 
=7r xlO x =about 4 square feet, 

corresponding to a heat loss on the above hypothesis of 
30 x 4 h.p. =120 h.p., which since the engine is only a 30 h.p. 
one is clearly too much. What is the cause of this dis- 
crepancy ? It is that the above elementary theory assumes 
that a considerable heat gradient is established immediately in 
the metal and that no heat is absorbed in the actual heating 
up of the layer of metal in contact with the gas. In point 
of fact if the temperature were suddenly raised to 400° C. 
a wave of heat would pass into the iron and before it had 
time to travel far the engine would have made a fraction of 
a stroke and the skin temperature have changed its value. 
The manner in which this wave of heat is formed has been 
worked out mathematically by Fourier and is given in his 
Analytical Theory of Heat. Some very interesting problems 
dealing with this subject have been worked out in Professor 
Perry's Steam Engine. 

57. A simplified treatment of the problem on mathematical lines 
maybe given here for the benefit of those who are acquainted with 
elementary differential equations. 

Let OO be the inner face of the section of the wall which can with 
sufficient accuracy for this problem be considered plane. Consider 



THERMODYNAMICS 



101 



what is happening at A, distant x below the surface of the metal. 
Across an imaginary unit area perpendicular to the surface of 
the paper and to the line of flow of the heat which is in the direc- 
tion of the arrow, heat will be transmitted but a part will be retained 
for the heating up of the substance of the lamina ft A. At a 
section at distance (x + 8x) the temperature will be (6 + 86), where 



Heat flow to 

*" Cooling water. 



oc 



Q 



Fig. 21. 

of course 86 is negative, at the same moment of time. Now the 

rate at which heat is received at the left face of the lamina contained 

d9 
by the two planes at x and (x + 8x) is equal to — k. -=- where k is 

UiX 

the conductivity and the additional amount which flows out per 
second on the other side is , { — k. , ).8x= — k.-j— 2 .8x. Now this 

heat must be equal to that required to raise the temperature of 
the lamina between the time t and the time (t + 8t), and the volume 
of the lamina being ( 1 X 1 X 8x) = 8x, it follows that the heat so 

absorbed must be equal to 8x.w. , .8t.s u where w= weight of unit 

volume and s 1 — specific heat. 
Therefore 

d9 



or 



d 2 6 
—k-, „.8x.8i 
ax" 

JL d*0 = d6_ 

w$ t ' dx 2 dt 



-tvs L . J 8x.8t 



(1) 



102 THE INTERNAL COMBUSTION ENGINE 

This is the equation for the flow of heat. The same equation occurs 
in problems relating to electric conductivity, to the diffusion of 
liquids into each other and into many other physical applications. 
Its solution is therefore well known, and in this case the simplest 
form of it is 

6= Ce— aX sin ( yi t — &x) (2) 

where C, a, y ± and fi are constants some of which can immediately 
be determined from equation (1). 
From (2) 

dO 

-£ = yi Ge—°*c6B{y 1 t—Px) 

dd 
— — = — aCe—** sin (yit — fix) — fiCe~ ax cos (y t t — fix) 

d 2 d 

-T- 2 = a 2 Ce— aX sm( yi t — fix) + afiCe—« x cosiyj — fix) + aflCf— ""COsfoS — fix) 

— fi 2 Ce~ ax sin (yit — fix). 
= (a 2 — /3 2 )<7e— ** sin (y 1 t—fix) + 2afiC<r- a * cos (yit— fix) 
So that equation ( 1 ) may be written 

— { (a 2 — fi*)C€—«* sin ( yi t—fix) + 2afiCe-« x cos (yit— fix) \ 

= yCe— aX COS (yit fix). 

For this to be an identity 

a 2 — / 3 2 =0 
k 
and 2aB — = 71 



WS-i 



V 2k 

As imaginary quantities are not wanted the positive value for a will 
be taken 

So that a=fi= V^" 1 

Substituting in equation (2) 



=^^"" n (^V^^) 



(3) 



58. Now when x = o the value of is that for the skin 
temperature of the metal — call this 
therefore # =C sin yit, 

and this suits the case in which the metal is infinitely thick 
and the temperature is measured from the mean temperature 
of the block as a zero. It shows a skin temperature which 
rises and falls as a simple harmonic function with an am- 
plitude of C, that is to say the range of temperature in 
the skin is 2(7. Now imagine the wall to be in contact with 
a highly heated gas the temperature of which fluctuates 



THERMODYNAMICS 103 

rapidly and unevenly. It is well known that by Fourier an- 
alysis this temperature can be represented by a series of simple 
harmonic functions of the time, of increasing frequency. In 
a gas engine the temperature of the gas rises and falls about 
a mean value in what is roughly a sine curve, and in any 
case the addition of two or three upper harmonics should 
make the representation very close. The effect of high 
harmonics at the interior part of the wall is, however, slight 
since it will be observed that the logarithmic decrement 
factor in equation (3) becomes more and more prominent as 
7 1 increases in value. It will therefore be sufficiently ac- 
curate in this analysis to consider the fundamental period 
only and to assume that it causes in the skin of the metal a 
fluctuation of temperature of much the same nature but of 
less amplitude and with at least some lag. What this 
amplitude and lag will be it is almost impossible to calculate, 
but some idea of the former has been gained by experiment 
which suggests that the range in the skin of the metal is in 
order of magnitude always less than one-third of that in the 
gas. Take, however, the extreme case in which the range of 
temperature in this skin is actually equal to that in the gas. 
This at least will represent the limit of what can occur in 
that direction. Then 6 = C sin y x t is the equation for the 
temperatures both of gas and skin. 

59. Effect at a depth. It remains to investigate how the 
rest of the metal wall is affected by this great vibration in 
temperature in one of its faces. It is clear from equation 
(3) that the amplitude decreases with the depth in the metal 
and that a lag arises and increases at the same time. The 

/wsiy-i qq 

amplitude at any point at a depth x is Ce ~W ' J but C 
is the amplitude at the surface, and therefore the fractional 
amplitude in the interior is e ~ ^/miux. 

It is of interest to evaluate this expression. 
We may put w= 450 lb. per cubic foot; t <? 1= 0'l: y x = 
IOtt for cast iron, taking the speed at 300 r.p.m. ; &=0'01. 

o ii ws.7, 450 

So that -^7^= - x01 xIOtt x 100 = 70.500 

2k 2 



104 THE IXTERXAL COMBUSTION EXGIXE 



or 



V 



2k 



266 



— 266x 



and the the fractional amplitude = e 
here of course x is in feet. If x be put equal to -f^ in. or 

^-g- foot, the fractional amplitude 



,5.53 



250 



or 0-40 of one 



per cent., which shows that even at a depth of only y-g inch 













































< 






















i 












































































8 






















CO 








1/ 












« 

1 
1 








/ 






















/ 



































































































js co »o y 9 ^* t" 

o <b 6 <b <b Q o 

'uotje/ftosQ ajnjejdduiaj. jo apn/i/duiy^y 



o 

£ p 

•° £ 

1 g 



<3 £ 






- c 



0^2 






o £ 

- z 



o 
M — 

J id 
I * 



THERMODYNAMICS 105 

the temperature oscillation is practically wiped out. The 
curve in Fig. 22 shows graphically how rapidly the oscilla- 
tions decrease in amplitude. So that in assuming the wall 
to be infinitely thick no very far-reaching assumption 
was made, since for anything over J in. in thickness the 
temperature on the water side will practically show no 
temperature oscillations,* and this even in so extreme a case 
as the above, where the inner skin is assumed to fluctuate 
through as great a range of temperature as does the gas 
itself. 

60. Practical Conclusions. This conclusion helps to 
simplify matters a good deal. It shows that the tem- 
perature gradient from face to face of the ivall is 'practically 
unaffected by the oscillation in the temperature of the gas, and 
that if to this sloping line, the above shallow temperature 
oscillations be added a representation can readily be ob- 
tained of what is actually occurring in the walls of a gas 
engine cylinder. The heat flow through the metal is known,, 
as regards quantity, from the heat balance-sheet for the 
engine, since the heat taken away by the cooling water must 
be exactly equal to the flow through the walls if a steady 
state has been reached. The difference in temperature 
between outer skin and water must just be enough to 
enable this amount of heat to pass. What this temperature 
difference may be is not certainly known. But the difference 
between the mean temperatures of the two faces of the 
metal can now be calculated. Thus in a 10 in. x 18 in. 
engine cylinder which loses 30 h.p. continuously through 
the walls, of an exposed area of 4 feet, the temperature 

gradient will be -i xl0=25° C. for a wall of 1 inch thick- 

ness. If the inner skin be at an average temperature of 
300° C. then the outer skin would be at 275° C, a value 
somewhat higher than that commonly supposed to occur. 



* Since the above was written Professor Coker has published the 
results of some actual tests made by him, showing that at a depth in 
the wall of $ inch the range of temperature fluctuation was only - J 
part of that in the inner skin. 



106 THE INTERNAL COMBUSTION ENGINE 

Prom the equations of par. 57 it is possible to calculate 
the flow of heat through the inner skin into the metal 
during the period of time in which the skin temperature 
is greater than that of the mass of the metal. Thus the 

heat flow at the surface must be — 1c( j 

\dx / X=Q 

= k,a.C - sin 7^ + cos yj, = h . a , C. J2 sin (y t t+-\ 

The amount of heat flowing per sq. ft. between times t=o 

and t =— must be = J y 1 lcaC>/Z sin fy.t + -j . dt = 

2kaC 

Inserting the values of the constants of the pre- 



7i 

1 v, u ±* » 0-01x266xC ftliy/1 

nous paragraph we have heat now =2- =0-176. 

Take for example a heat equivalent to 7 J h.p. per sq. ft. of 

surface, then heat flow in one revolution (at 300 r.p.m.) = 

550 

7-5x x0-2=0-59 C.h.u.. which may equate to 0-17C. 

1400 l H 

giving (7=3-5 deg., or a total temperature oscillation of 7 
degrees.* This indicates generally that a small oscillation 
in the temperature of the innermost layer of metal is quite 
sufficient to absorb and level the temperature oscilla- 
tions which the gas tends to set up in the cy Under walls. 
One may conclude from this that although the temperature 
at any point of the walls may depend on the position of 
that point in the cylinder it does not sensibly vary with the 
time, that is to say,, that at any given point the temperature 
in the walls remains nearly steady. The wall may there- 
fore be considered as of two parts — the inner skin which 
acts as an accumulator of heat energy, rapidly abstracting 
it during explosion and giving it out again later ; and 
another part, consisting of the whole of the rest of the 
wall, which acts as a steady transmitter of the heat fed 

* Since this was written Professor Coker has published some ex- 
perimental work he has done in which he found that the maximum 
skin temperature was not more than 4 = C in excess of the mean. 



THERMODYNAMICS 107 

into it through the inner layer. These considerations may 
go some way to throw light upon the problem of the cool- 
ing effect of the walls of a gas engine cylinder. 

PROBLEMS. 

1. How are the pressure, volume and temperature con- 
nected in a perfect gas ? 

Dry air is pumped into a closed vessel of constant volume 
until that pressure is 80 lb. per square inch by gauge, the 
temperature being 90° F. What will be the pressure in the 
vessel after it has remained for a considerable time in a room 
where the temperature is 60° F. ? Ans. 74*9 lb. /in. 2 by gauge. 

(Mech. Sc. Tripos, Part II, 1906.) 

2. Gaseous stuff has a specific heat K, at constant pressure, 
of 26 ; and a specific heat k at constant volume of -190. 
Joule's equivalent being 1393 : 

(1) What is its law of adiabatic expansion ? 

(2) If a pound of it is at 120° C, pressure 5,000 lb. per 

square foot, what is its volume ? 

(3) A pound of it expands according to the law pv s con- 

stant. What is its rate of reception of heat ? 
Ans. 7-65 cu. ft. and (18500—13505). (B. of E., 1899.) 

3. Fluid expands from a point on the diagram where p 
is represented by 1-5 inches, and v by 1 inch, to a place 
where v is 3 5 inches. According to each of the laws of 
expansion p v constant, p v v0M6 constant, and p v 1 ' 13 
constant, find the value of p at the end of the expansion 
in each case. Ans. 0'428 ; 0395; 0'376 ins. 

(B. of E., 1900.) 

4. The law of cooling in Dugald Clerk's gas explosion 

31 
experiments is p = — =- where t is time in seconds and p is 

v t 

pressure in lb. /in. 2 . Calculate the rate of loss of heat per 
second, i.e. — . Given that C v =/3 -{-sO. 

Ctv 

5. In a gas engine diagram the expansion curve usually 
lies above the adiabatic expansion curve, showing that if 



108 THE INTERNAL COMBUSTION ENGINE 

the working substance be a perfect gas it must be receiving 
heat during the expansion, yet in fact much heat is with- 
drawn from the cylinder walls by the cooling water. What 
do you regard as the most probable explanation of this ? 
Give some account of the arguments and experimental 
evidence which lead you to prefer your explanation to others 
that have been adduced. 

(Mech. Sc. Tripos, Part II, 1904.) 

6. A gas expands so that pv n is constant. Show that if 
n is equal to the ratio of the specific heat at constant pres- 
sure to the specific heat at constant volume the expansion 
is adiabatic. (Mech. Sc. Tripos, Part I, 1898.) 

7. The piston of an air compressor displaces 8 cubic feet 
per stroke and makes 120 strokes per minute. It takes in 
atmospheric air at 60° F. and compresses it, according to 
the law PF 1-25 =constant, up to 75 lb. gauge pressure; 
finally delivering it at this pressure into a reservoir. As- 
suming no slip past the valves, no loss of head through 
them, and in the first place no clearance, calculate the work 
done upon the air in foot-pounds per minute, and the tem- 
perature at which it enters the reservoir. In the second 
place, if the clearance were 10 per cent, of the piston dis- 
placement, how would the work done per minute and the 
volumetric efficiency of the compressor be affected ? At- 
mospheric pressure =14-7 lb. per square inch. 

Ans. 2-26 cu. ft., 19'8 B.T.U. and — 0083. 

(Mech. Sc. Tripos, 1906.) 

8. In a gas engine release occurs at seven-eighths of the 
stroke and at a pressure of 40 lb. per square inch absolute. 
The clearance space is a quarter of the total cylinder volume. 
The engine works on the Otto cycle, explodes every time 
and is not scavenged. The mixed gas and air just before 
being drawn into the cylinder on the suction stroke has a 
temperature of 100° C. Estimate the temperature of the 
charge filling the cylinder at the end of the suction stroke. 

In making your estimate you will probably assume that 
gases before and after explosion behave as the same perfect 
gas. How far is this assumption correct ? Illustrate the 
possible errors in estimates of temperature based on this 



THERMODYNAMICS 109 

assumption by finding their amount in the case of a mixture 
of one volume of hydrogen and five of air. Ans. 3 4 and 
3-8 x 10 6 ft.-lb./min. (Mech. Sc. Tripos, Part II, 1904.) 

9. It is found that the area of the diagrams of the 
Crossley engine in the laboratory averages about 20 per 
cent, bigger when the engine is running light than when it 
is fully loaded. Explain this. 

When the engine is running fully loaded the temperature 
of the exhaust gases left in the clearance space at the end 
of the exhaust stroke is 700° C, and the temperature of 
the gas and air sucked in just before they enter the cylinder 
is 100° C. The clearance space is a quarter of the total 
cylinder volume (including clearance space). Show that 
the temperature of the gases filling the cylinder at the end 
of the suction stroke will be 170° C. Assume that no heat 
is lost to or gained from the cylinder walls during suction, 
that the pressure inside the cylinder is the same as that of 
the atmosphere, and that the specific heat of the exhaust 
gases, and of the incoming charge is the same constant 
quantity. (Mech. Sc. Tripos, Part I, 1904.) 

10. The equation for the flow of heat in the walls of an 
engine cylinder may be assumed to be 

k ^V _ $V 
~cJx?~~S^ , 
k and c being the conductivity, and thermal capacity per 
unit volume, of cast iron, and V the temperature at time t 
at the distance x from the surface. Show that the solution 
of this equation is V = Vie~ mx cos(0 — mx) for a simple harmonic 
variation of surface temperature of semi-range V± ; being 
the angle described by the crank in time t, its angular 
velocity being 2irn. 

Hence find the range at any depth x ; prove that the 
value of the index co-efficient, m, is V(irnc/k) ; and find 
the heat absorbed in thermal units per square foot of Avail 
surface per period, the semi-range at the surface being TV 

(Mech. Sc. Tripos, 1906.) 

11. Prove the formula for efficiency in the hypothetical 
Otto Cycle (the diagram being two adiabatics and two 
constant volume lines), showing how efficiency is greater as 



110 THE INTERNAL COMBUSTION ENGINE 

clearance is less. In what way does this hypothetical 
diagram differ from reality ? If it differs greatly, why are 
such calculations of any use ? (B. of E., 1906.) 

12. Given a gas engine diagram, show how we may draw 
a diagram showing the rate (1) per cubic foot change of 
volume, (2) per second, at which the stuff receives heat from 
the beginning of the compression to the release. 

You may assume an infinitely long connecting rod. 

(B. of E., 1900.) 

13. An air compressor pumps air in a steady stream into 
the lower part of a reservoir against a pressure of 500 lb. 
per square inch absolute. The reservoir is partly filled with 
water and is maintained, by an outside source, at a constant 
temperature of 300° F. An equal stream of air passes 
from the upper part of the reservoir through a reducing 
valve, carrying with it water vapour and suspended water 
and goes to supply a system of air motors. Find what 
portions of the pressure in the reservoir are due to air and 
water vapour respectively, and how many pounds of air 
are mixed with each pound of vapour in the upper part of 
the reservoir. 

If the pressure beyond the reducing valve is p lb. per 
square inch absolute, write down the equations which 
determine the state of the mixture after passing the valve, 
assuming that each pound of vapour carries 3 lb. of 
suspended water, and that the vapour does not become 
superheated. 

14. A mass of unequally heated perfect gas is enclosed 
in a vessel whose walls are impervious to heat. Prove that 
the pressure of the gas remains unchanged during the 
equalization of the temperature by connexion and con- 
duction. (Mech. Sc. Tripos, Part I, 1904.) 

15. Air flows through an orifice from a reservoir in which 
the pressure is p lb. per square foot and temperature t into 
a region of lower pressure — heat being neither received nor 
rejected during the operation. Obtain an expression for 
the maximum discharge in pounds per second, in terms of 
p, t, the effective area of the orifice, and the ratio of the 
specific heats. 



THERMODYNAMICS 1 1 1 

Are the general conclusions arrived at by theory verified 
in practice ? If not, state what the experimental results 
obtained really are, and point out where the theory probably 
fails. (Mech. Sc. Tripos, Part II, 1904.) 

16. Air is compressed adiabatically into a receiver of 
V cubic feet capacity to m times the atmospheric density. 
Show that if p be equal to the atmospheric pressure, the 
work expended is 



F(^ mY -^ m +l)foot-lb. 



V 

r 

y being the ratio of the specific heats of air. 

(Mech. Sc. Tripos, Part I, 1904. 



APPENDIX 



As this book goes to press, the Gaseous Explosions Committee 
of the British Association publish their first report. After con- 
sidering all the available specific heat measurements they pre- 
sent a curve of C v and 0, which they consider to represent values 
accurate to 5 per cent. The following equation fits this curve 
very well. 

n 

C v = 0172 + 0075 j^ 
This would make equation (14) become 

W 1 - mo a=v.T 2 +4M)} 

and equation (15) 

, 0=0 .60(0.90- -Jy . 

Also it is worth noting that the efficiencies of par. 47 would 
become 0-43 and 0-46 in place of 0-44 and 0-46. 

The further report of the Committee will be awaited with much 
interest. 



SECTION II 
GAS ENGINES AND GAS PRODUCERS 



CHAPTER V 

The Gas Engine 

Types of Gas Engine — Methods of Improving their Efficiency 
— Indicators, Old and New — Heat Balance Sheets — 
Engine Tests — Governing — Cyclic Irregularity — 
Balancing. 

61. Types of Gas Engine. The several thermodynamic 
cycles upon which gas engines are capable of working have 
already been described, but it may be said at once that 
practically all gas engines now being built are designed to 
work on the constant volume cycle, or as near thereto as can 
be effected. Students will also hear of other cycles such as the 
Otto and the Clerk cycles. These names refer to the cyclic 
operation of the exhaust and inlet valve gear and not to the 
thermodynamic ideal to which it is desired to make com- 
bustion conform. The Otto cycle consists of four strokes : 
the admission stroke when the piston is moving outwards, 
the compression stroke when it returns, the expansion 
stroke which occurs after explosion has taken place, and 
the fourth stroke, generally known as the scavenging stroke, 
when the burnt gases are pushed out of the cylinder. As 
each cycle includes two revolutions of the engine the valves 
are operated from a cam shaft which rotates at half the 
speed of the main shaft and is therefore called the half- 
time-shaft. Examples of the Otto cycle are found in nearly 
every type of engine now built. In the Clerk cycle there are 
only two strokes, the explosion stroke and the compression 
stroke. This cycle will now be described in greater detail. 

In addition to the actual engines illustrated in this 
chapter the author shows, also, illustrations of the more im- 
portant engine details, for many of which he is indebted 
to the kindness of the National Gas Engine Co. ; and for 

115 



116 THE INTERNAL COMBUSTION ENGINE 



others, to the several makers of the engine types described. 
62. The majority of gas engines * at present in use work 
on the Otto cycle, but a considerable number of the larger 
I 




Fig. 23. — Exhaust valve of a 
National Gas Engine. Mush- 
room type. Four Cycle Engine. 



Fig. 24. — Inlet valve of a National 
Gas Engine. Mushroom type. Four 
Cycle Engine. 



sizes of engine do not. Among the latter are the Koerting 
engine, made in this country by Messrs. Mather & Piatt ; 
and the Oechelhauser engine, which is manufactured in 
England by Messrs. Beardmore. Illustrations of both 
these types are shown. In each case the exhaust passes 
through ports in the cylinder walls which are overrun 

* All are water- jacketed. It is only the smallest petrol engines 
that rely on air cooling. 



THE GAS ENGINE 



117 



by the moving piston. The Koerting piston is made long 
and the ends of the cylinder are coned. At these coned 
ends are placed the inlet valves, through which the working 
charge is 'pumped by two pumps, one for air and one for 
gas. The engine is a double-acting one and every stroke is 
a working stroke just as in a steam engine. The great length 
of the piston prevents the exhaust ports being overrun until 




Fig. 25. — Admission end of Cylinder of National Gas Engine. Note the 
conical interior and fitting of cylinder liner. Note also the cams on 
half-time-shaft for operating valves. Four Cycle Engine. 

the end of the stroke, whether the piston is moving from left 
to right or right to left. Once the exhaust port is uncovered 
the gases pass away, and are helped on their passage 
by the air which is then being admitted in the coned ends 
of the cylinder. When this has gone on for a short time, 
gas is admitted also and the mixture is ready for compression 
on the further motion of the piston. The time taken for 



118 THE INTERNAL COMBUSTION ENGINE 

the crank to pass the few degrees of slow piston motion on 
each side of the dead centre affords opportunity for these 




c3 <*h 



S -a 



&H © 

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I 5 

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^ -^ © 

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'5c £ >> 



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.2 pi 

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PI h 

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IV, O 



THE GAS ENGINE 



119 



operations to take place. An obvious difficulty about this 
method of working is that some of the incoming gas may be 
caught up and pass away with the exhaust products and so 
be lost. This reduces economy, but is of little impoitance 
when working with what are known as waste gases*, as 
these are produced in immense volume and at practically no 
cost. Attention has been given in the previous chapter to 
the question of piston and cylinder wall temperatures, and 
it will therefore be readily understood that in such a cycle 




Fig. 27. — Cross Section of Two Cycle Engine (Koerting) — new type, by 
Messrs. Mather and Piatt. Compare with Fig. 26. 

as this the heating effect of explosions so closely following 
each other will be severely felt and high temperatures 
are likely to be reached by all parts open to the gases. 

63. The Oechelhauser engine resembles the Koerting inas- 
much as it has ports in the cylinder walls which are over- 
run by the pistons, but the method of working is quite 
different, as in this type each cylinder has two pistons which 
move inwards and outwards together, so producing a well- 
balanced motion. The joint centre of gravity of the two 
pistons does not move. One piston operates directly on 
its crank and the other through return connecting rods on 
to another crank placed 180° from the former one. As will 
be seen from the illustration (Fig. 31), the piston on the left 
overruns first the exhaust ports, thus enabling the exploded 
gases to leave the cylinder. This operation is then helped 
by the right-hand piston overrunning the air inlet ports 
so that air rushes in and aids the exit of the waste products. 
* See Chapter VII. 



120 THE INTERNAL COMBUSTION ENGINE 




THE GAS ENGINE 



121 



Then the right-hand piston in its further motion overruns 
the gas inlet ports and so a proper mixture is admitted, 
which, when the two pistons move in together, gets com- 
pressed for the next explosion. This gives an explosion for 
every time the pistons separate, or an explosion every revolu- 
tion. The engine is provided with separate pumps for the 




Fig. 29.— Cross Section of 400 B.H.P. Koerting Engine by 
Mather and Piatt. 

air and gas. That there are many difficulties to be over- 
come in the construction and working of large gas engines, 
particularly those working on the Clerk cycle, is shown 
by the unfortunate history of the generating station for 
lighting and tramway work which was established some 
years ago at Johannesburg. The plant was operated with 
Oechelhauser gas engines driven by gas from Poetter 
producers using Transvaal coal. The electric portion of the 
plant consisted of Siemens generators, giving a total output 
of 8,100 K.W. From the very beginning many difficulties 
arose in the operation of this plant, mainly on account of the 



122 THE INTERNAL COMBUSTION ENGINE 

lack of requisite experience in the manufacture and opera- 
tion of similar installations. The cost per unit is reported 





\mi mBa. 


\. ill 


M 


Mil 




\\ p <im "\ 


SpW '?IL ' 


,;:".f: |P^\i|i 


A 111 If 

9hRk 




1V»vmTi 1\\ 


^^^™ 


Mil 



to have been higher than was originally anticipated, and it 
is understood that the plant has since been shut down. 



THE GAS ENGINE 



123 



This occurrence must 
not be taken to 
show that Oechel- 
hauser engines are 
unsuitable for heavy 
work, since in 
numerous places on 
the Continent engines 
of this make are 
working well and 
giving entire satis- 
faction to their 
owners. The largest 
installation of gas 
engine plant in this 
country is at pre- 
sent that at the 
Cargo Fleet Iron 
Co.'s works at Mid- 
dlesbrough. It con- 
sists of six Cockerill 
engines, built by 
Messrs. Richardsons, 
Westgarth & Co., of 
900 h.p. each, or a 
total of 5,400 h.p. 
These engines work 
on the Otto cycle, 
but by having 
double - acting tan- 
dem cylinders the 
crank gets just as 
many impulses as in 
a double - acting 
steam engine. 

64. Having now 
gained some idea of 
the manner of work- 
ing of a few of 




124 THE INTERNAL COMBUSTION ENGINE 




THE GAS ENGINE 



125 



£== 




Fia. 33. — Arrangement of Valve Gear for Cockerill Engines (Richardsons, 
Westgarth and Co.). Note position of cam shaft for actuating inlet 
and outlet mushroom valves. 



126 THE INTERNAL COMBUSTION ENGINE 

the more important types of large gas engines, the 
student may well turn his attention more closely to the 
origin of the Clerk or two-stroke cycle. That it is a very 
important cycle is obvious, bearing in mind the necessity 
of keeping down the capital cost of gas engines per horse- 
power. The Clerk cycle owes its origin to the inventive- 
ness of Mr. Dugald Clerk, and the writer cannot do better 
than quote the inventor's remarks upon it before the 
Society of Arts in 1905 : "In the Clerk engine the motor 
cylinder had, at the front ends, large ports leading into an 
annular space, these being the exhaust ports. The com- 
pression space was conical, and the charge was sent in by 
means of a separate pump, which I called the displacer. 
The action of the engine was as follows : When the piston 
got to the out end of its stroke, and the crank was crossing 
the out centre, the piston overran the exhaust ports on the 
out-stroke, and covered them on the in-stroke. Meantime 
the pump or the displacer piston, which was attached to a 
crank at right angles in advance of the main crank, was 
sweeping in and giving its charge a slight compression. 
That charge passed through a connecting pipe, and through 
a check valve, into the conical end, displacing before it all 
the contents of the cylinder. When the main crank had 
returned about 40 degrees of its circle under the centre, 
these ports were closed. It opened about 40 degrees above 
and closed 40 degrees under, and in that time the displacer 
piston had gone fully in and discharged its charge into the 
cylinder and combustion space through the lift valve. Then 
the motor piston compressed the charge, and ignition took 
place at the in-end of the stroke, just as in the Otto cycle. 
The object of the invention was to enable one motor cylinder 
to give an impulse at every revolution. In the Otto cycle 
there is only one impulse for two revolutions, so far as the 
main cylinder is concerned. The Clerk engine gave one 
impulse for every revolution of the main crank in the main 
cylinder, but to make that possible it was necessary to 
provide an auxiliary crank and displacer cylinder. The 
idea, was, of course, to diminish the irregularity of the Otto 
cycle by having an impulse at every revolution, or more 



THE GAS ENGINE 



127 




128 THE INTERNAL COMBUSTION ENGINE 

frequently, that is to say, two impulses per revolution, 
obtained by making the engine double-acting. The object 
was to get very much more power for a given weight of 
engine, as the pump was light and only required to deal 
with its charge at a low pressure. In construction the engine 
was a very simple one." 

65. Other Types of Engines. The Ehrhardt and Sehmer 
engine works on the four-cycle double-acting principle, giving 
an explosion per revolution for each cylinder, so that for two 
cylinders placed tandem every stroke is a working stroke just 
as in a steam engine. With the tandem arrangement there 
is the advantage that the motion towards each dead centre 
is always preceded by a compression stroke ; this leads to a 
cushioning action which is useful as an aid to overcoming 
the inertia effects due to the moving parts. These engines are 
very effectively water-cooled ; the parts so treated include 
the pistons, piston rods, cones, glands, exhaust valves and 
valve casings. The engines are stated to be capable of 
driving alternators direct and allowing of parallel running. 
M. R. E. Mathot * writing in Gas and Oil Power, stated 
that : "A 600 h.p. double-acting engine, with two tandem 
cylinders, installed at the Kgl. Berginspection at Heintz, 
Saarbruck, Messrs. Ehrhardt and Sehmer, was tested at the 
end of last year, without cleaning, and after four months' 
•continuous work with coke-oven gas of from 4,000 to 4,200 
calories ; the trial was carried out by the makers' engineers 
under the supervision of Kgl. Berginspection's engineer. 
Thanks to a large gasometer, the record of gas consumption 
was taken for a period of one hour. The mechanical 
efficiency was 83 per cent. The engine was new and was 
tested on a normal load. The dimensions of its pistons and 
piston rods were respectively 620 mm. and 170 mm., 750 mm. 
stroke, and 150 revolutions. It just reached 520 K.W., 
generating three-phase current. In the conditions of the 
trial the actual thermal efficiency was more than 31 per 
cent., or nearly 37 J per cent, of that indicated. Unfortu- 
nately, similar trials to this are rare because gasometers 
are not usually sufficiently large to measure the exact amount 
* December 15, 1906. 



THE GAS ENGINE 



129 




130 THE INTERNAL COMBUSTION ENGINE 

of gas used by the engine." Another well-known type is the 
Premier gas engine. Perhaps its most familiar feature is the 
scavenging of the exhaust products by means of an air 
blast. It has been stated that this blast is capable of 
keeping the cylinder interior almost entirely free from 
deposit when working with bituminous fuel gas plant. 
An account has been published * of a 1,200 h.p. 
four-cylinder gas engine by the Premier Gas Engine Co. 
which was constructed for direct coupling to a con- 
tinuous current generator and consisted of two sets of 
tandem cylinders working on cranks set at 180 degrees 
apart. A four-stroke cycle was employed, so there were 
two working strokes per revolution. A scavenging charge 
of air supplied from a separate air cylinder at about 3 lb. 
per square inch was used to clear the cylinders of waste 
products. All the valves were placed on the cylinder 
covers ; pistons and exhaust valves were both water-cooled. 
It was stated that, operating on producer gas, a compression 
pressure of 140 lb. per square inch could be used without any 
difficulty whatever from pre-ignition, and that a test on 
the engine showed the mechanical efficiency to be as much 
as 87 per cent. The Westinghouse Co. mainly produce a 
vertical engine, and are among the first to do so. Their 
250 h.p. type has three cylinders and in general appear- 
ance resembles a high-speed vertical steam engine with 
its boxed-in crank chamber and splash lubrication. They 
also use the Otto cycle, and it is claimed that the governing 
is sufficiently steady to admit of the driving of alternators 
operated in parallel. The number of types of engines of 
less than 400 h.p. is very great, and the number of varieties 
increases as the output gets less than this. Of the smaller 
engines nearly all work on the Otto cycle and illustrations 
are shown of several types. 

66. Methods of Improving Efficiency (Crossley and Na- 
tional). — The author has already endeavoured to make 
it clear that one of the chief causes why the efficiency of 
gas engines is not even higher than it is, lies in the high 

* Engineering, January 11, 1907. 



THE GAS ENGINE 



131 



temperatures which occur during explosion and the very 
rapid rate at which heat is then abstracted by the walls. 
Two methods have been practically tried with a view to 




Fig. 36. — Standard Arrangement of Campbell Gas Engine. Note the 
disposition of the inlet and exhaust valves, and the water cooling ar- 
rangements. On the plan at the lower side is seen the half-time-shaft. 

minimize this effect, the idea in each case being to reduce 
the maximum temperature of the cycle without, however, 
decreasing the mean pressure. These two are the water 
injection method of Messrs. Crossley Bros., and the super- 



132 THE INTERNAL COMBUSTION ENGINE 



compression method of Mr. Dugald Clerk and the National 
Gas Engine Co. 

The Water Injection Method. — Messrs. Crossley decided 
to try the effect of injecting a small spray of water into 
the cylinder during the suction stroke. The water, enter- 
ing as a fine spray in part of the air supply was as evenly 
distributed as possible and did not form a water film on 




Fig. 37. 



-Typical Gas Engine Piston and small end of Connecting Rod. 
Note method of lubricating small end. 



the cylinder walls. Very little water is required, because of 
the high value of its latent heat. As the mixture explodes 
the water mist is evaporated into steam and the heat so 
absorbed prevents the temperature of the gases from rising 
unduly high. A 50 h.p. engine so adapted was tested by 
Professor Burstall in 1904 and the following records taken : — 



Size of 


engine 


14 in. x21 in. 


Duration of test 


6h. 45 m. 


Average 


) revs./min., . 


16602 


,. 


explosions /min . 


81-2 


J5 


mean pressure lb. /in. 2 . 


91-44 


>J 


i.h.p. . 


60-5 


JJ 


b.h.p. . 


49-7 


Mechanical efficiency . 


82-2 per cent. 



THE GAS ENGINE 



133 



Gas used per i.h.p.-hour . . 11*77 cu. ft. 

„ b.h.p.-hour . . 1443 

Calorific value of gas (lower value) 578 B.T.U. per cu. ft. 



Thermal efficiency on i.h.p. 
„ b.h.p. 
Water used in cylinder 

„ discharged from jacket 
Average rise in temperature of 
jacket water .... 
Mean temperature of exhaust as 
measured by Callendar pyro- 
meter ..... 



37-43 per cent. 
30-8 

0131 lb. per minute. 
25-66 

77-52° F. 



718° F. 




Fig. 38.— Fielding Gas Engine. 290 B.H.P. Two-cylinder-side-by-side 
type. Note the lubricating arrangements. 

The ratio of air to gas was 10 -2 and the compression ratio 
8 7 (obtained by dividing the clearance volume of 243 
cu. ft. into the cylinder volume of 1-872 cu. ft.). 

Now a compression ratio of 8 7 corresponds on the " air 

/ 1 \ ' 4 
standard" to an efficiency of 1 — I ^ J which equals 

058, and as the actual efficiency found was 037 it follows 



134 THE INTERNAL COMBUSTION ENGINE 

that the engine achieved nearly 64 per cent, of the " air 
standard " efficiency. This is a higher ratio than any of 
those given by Mr. Dugald Clerk in his 1907 paper before 
the Institution of Civil Engineers (" On the Limits of Ther- 
mal Efficiency in Internal-Combustion Motors "), which 
showed no higher ratio than 59 per cent, and that only in 
the case of a maximum temperature of 1,098° C, whereas 
when the temperature rose to 1,750° the ratio fell to 50 per 
cent, and below. On this method of comparison, therefore, 
the water injection method shows to advantage. 

The Super- Compression Method.— This method is due to 
the ingenuity of Mr. Dugald Clerk, who in his James Forrest 
Lecture (1904) before the Institution of Civil Engineers, 
described it thus : " Some time ago it appeared to me 
possible to reduce maximum temperatures by increasing 
the charge-weight per stroke given to an engine. I had 
experimented with two engines, one having a 7 in. cylinder, 
15 in. stroke, and the other a 10 in. cylinder, 18 in. stroke. 
These engines, which are of the ordinary standard four- 
cycle type, are allowed to take in the usual charge of gas 
and air ; then at the end of the stroke a further charge of 
air or other inert fluid is added to increase the pressure in 
the cylinder to 7 lb. or 8 lb. per square inch above atmosphere 
before the return of the piston. A small part of the return 
stroke is, however, made before the pressure can be ma- 
terially increased as the added charge takes some time to 
fill the cylinder. This has the effect of increasing the 
charge weight present in the cylinder by about 40 per cent, 
and of increasing the pressure of compression without, 
however, increasing the temperature of compression. 
Indeed in both experiments the temperature of compression 
was diminished. As the charge present is constant so far 
as gas is concerned the maximum temperature capable of 
being produced is much reduced. The maximum tempera- 
ture shown by the diagrams taken by me from these two 
engines is about 1,200° C. Experiments were made and it 
was found that the heat-flow was reduced to about two- 
thirds, and further that the mean available pressure was 
increased about 20 per cent." 



THE GAS ENGINE 



135 



The thermal efficiency of an engine which on working 
without super-compression was 27 7 per cent, showed an 
increase to 34-4 per cent, when super- compression was 



w 

1 m 


^M0 


II 1 

11 m\ 

I Ml 1 
H hEUS m 







adopted. One sees therefore that if the atmospheric 
pressure were 50 per cent, higher than it is, it would 
suit the working of gas engines a great deal better. 

The increases in thermal efficiency obtained by the water 



136 THE INTERNAL COMBUSTION ENGINE 

injection and the super-compression methods are of course 
desirable in themselves, but they are really the most welcome 
for what they bring in their train, viz. freedom from cracking 
of cylinders and pistons. Low efficiency means a large 
amount of heat being passed away through the walls to the 
cooling water, and the larger the engine the larger the amount 
of heat to be got rid of in this way and the smaller in propor- 
tion to cubic contents does the cooling surface become. This 
means a steep heat gradient in the metal, and this in turn 
leads to the failure of engines owing to the cracking of 
ends or walls or sometimes of pistons themselves if water 
cooled as is usual in the larger sizes. Manufacturers may 
therefore be said to be seeking high efficiencies not for the 
resulting economy in fuel (for the gas engine has there long 
left all competitors behind) but on account of the increased 
freedom from mechanical difficulties in operation. It is a 
particularly happy feature of the case that gas engine 
builders seeking and obtaining increased reliability of opera- 
tion should also find that the same improvement leads also 
to greater fuel economy. Economy in fuel is of no great 
direct importance in the case of large engines working on 
blast furnace or coke-oven gases, but in other cases it is 
often an advantage. Improved methods which allow of 
the maximum cyclic temperature being reduced without any 
loss of power can also be pressed in the direction of increasing 
the mean pressure considerably without, however, raising 
the temperature so high as it was previously. This leads 
to greater output, but the pressure at exhaust is considerable, 
and it would in such cases be an advantage to use this 
exhaust in another cylinder and so compound the engine. 
Efforts in this direction are being made. 

67. The Indicator. — A very important instrument used 
in connexion with gas engines is the indicator. It is an 
apparatus which when attached to an engine draws a 
curve showing how the pressure in the cylinder varies at 
different points in the stroke. The best known modern form 
is the Crosby shown in Fig. 41. On the left of the illus- 
tration will be seen a small cylinder containing a cup-shaped 
piston which is regulated in its upward motion by the down- 



THE GAS ENGINE 



137 




138 THE INTERNAL COMBUSTION ENGINE 

ward push of the strong spring seen above. When the 
indicator is screwed on to the engine cylinder the pressure 
causes the indicator piston to rise through a distance pro- 
portional to the force exerted. The little piston rod rises 
also and communicates its motion to the long sloping lever 
seen above. This lever carries at its far end a pencil which 
traces a line on a paper sheet fastened round the drum seen 
on the right which is made to oscillate to and fro by the 




nil 



Fig. 41. — Crosby Indicator with 
Internal Spring. 



Fig. 42. — Crosby Indicator 
with External Spring. 



cord shown on the extreme right of the diagram being 
attached to some part which has a motion similar to that of 
the piston, but less in amount. The pencil therefore traces 
out the closed curve known as indicator diagram. These 
diagrams will be familiar to all readers of this book and 
probably the indicator also — if not it would be well worth 
any student's time to spend some hours examining a Crosby 
instrument and taking readings with it. 

Fig. 42 shows another form of the instrument having 
the spring outside where it is less affected by the heat and 
so gives a better reading. Of course all these springs are 
carefully calibrated first so that it is known how much 
pressure is represented by a rise of the pencil point equal 
to, say, 1 inch. There are certain qualities which a well- 



THE GAS ENGINE 



139 



designed indicator should have. It must have a spring stiff 
•enough to ensure that the maximum pressure will come 
well within its range. It must have a well-designed piston, 
as light as is consistent with strength, which will move 
freely in the cylinder. A slight leakage of steam is much 
less of an evil than any chance of the piston sticking or 
jambing. In the Crosby form the piston is made from a 
solid piece of tool steel, hardened and then ground and 
lapped to gauge. It is provided with a socket to receive 
the bead at the end of 
the spring and has screw 
adjustments for locking 
the spring in place. The 
rod is made hollow for 
lightness and is threaded 
at its lower end for 
attachment to the piston. 
The spring is made with 
a double coil so as to 
centre it the better. It 
has already been men- 
tioned that the to and fro 
motion of the paper is ob- 
tained from a cord at- 
tached at its other end to 
some point in the upper 
part of a swinging lever 
of which the lowest point 
is connected with the engine piston or some part that moves 
with it so that the motion of the engine piston is reduced 
to a convenient degree. There is also a Crosby reducing 
device for doing this in a simpler way. It is shown 
illustrated in Fig. 43 and its principle of action is easily 
seen therefrom. In this device the cord at the bottom 
can be fastened direct to the crosshead, or other part 
attached to the piston, the cord passing over guide pulleys 
if necessary. It is better, however, not to have a longer 
cord than necessary as its stretching with the pull put 
on it may introduce serious error into the indicator card. 




Fig 



-Reducing Gear ior 
Indicator. 



140 THE INTERNAL COMBUSTION ENGINE 

For very accurate work the cord has sometimes been 
replaced by steel wire. 

68. Reflecting Types of Indicator. Although as has 
been stated the indicator is a very important instrument 
in gas engine work, it does not nevertheless occupy the 
important position it does in steam engine practice 
where lower maximum pressures are met with. This may 




Fig. 



44. — 360 B.H.P. Vertical Four- cylinder Campbell Gas Engine, 
coupled to Electric Generator. Note position of cam shaft. 



be due to the fact that most of the steam engine work 
was done at a time when the unavoidable errors of the 
indicator instrument were less well realized. Latterly it has 
become common to record the motion of the indicator piston 
by means of a beam of light reflected, as in a reflecting 
galvanometer, from a vertical mirror which is caused to 
tilt as the piston rises ; at the same time the frame in which 
the mirror is held is made to move angularly to and fro 
in time with the motion of the crosshead, thus producing 
by the combination of motion the familiar shape of the 
indicator card. The beam of light, unlike the steel levers 
of the older form of indicator, has no weight and therefore 



THE GAS ENGINE 141 

no inertia to make it lag behind its true position. Professor 
Hopkinson claims that with an instrument devised by him 
on the reflecting principle, the indicated horse-power can be 
measured with an error of less than 1 per cent., whereas in 
the older forms errors of 5 per cent, or more were common. 
Professor Hopkinson * in a paper read by him recently 
made the following comment : — 

" In the report of the Committee of the Institution of 




Fig. 45. — Air Pressure Vessel and Pumps for starting 
Campbell Gas Engine. 

Civil Engineers on the Efficiency of Internal Combustion 
Engines the following remark occurs : ' It would be desirable 
but for one circumstance to calculate the relative efficiency 
only from the indicator horse-power. But it appears that 
in the case of gas engines, and especially gas engines 
governed by hit-or-miss governors, the indicator diagrams 
do not give as accurate results as is generally supposed. 

* "On the Indicated Power and Mechanical Efficiency of the 

Gas Engine," Institute of Mechanical Engineers, 1907. 



142 THE INTERNAL COMBUSTION ENGINE 

The diagrams vary much more than those of a steam engine 
with a steady load, and the mean indicated horse-power, 
from the diagrams taken in a trial, may, it appears, differ 
a good deal from the real mean power.' This statement is 
fully borne out by the tests of the Committee, which show 
that the mechanical efficiency taken as the ratio of brake 
to indicated power varied from 80 per cent, to 94 per cent, 
in the three engines tested. These engines were of similar 
type, but of different sizes, and whereas the smallest of 
5 h.p. showed a mechanical efficiency of 90 per cent., the 
intermediate engine of 20 h.p. showed a lower efficiency of 
80 per cent. The Committee remarked that these values 
were obviously incorrect, and the values adopted by them 
for the mechanical efficiency were obtained by running the 
engine light and making an estimate of the indicated horse- 
power under these conditions. Assuming that the mechani- 
cal loss is constant at all loads, the indicated power at full 
load can be determined by adding the power absorbed at 
no load to the brake-power. The mechanical efficiencies of 
the three engines found in this way were respectively : 
Engine . . . . L R X 

Mechanical efficiency . 0-86 0866 0-888 

" These results are just what would be expected ; the 
mechanical efficiency showing a slight improvement with 
the size of the engine." 

69. The method of getting the i.h.p. by assuming that 
the mechanical loss is constant at all loads is hardly satis- 
factory. It is obvious that to assume that the friction at, 
say, the big and small ends of the connecting rod, or on 
the piston, will be the same whether there is any thrust in 
that rod or not cannot be strictly accurate, and even if the 
result of making this assumption is to produce a result 
reasonable in itself, that is not a valid reason for accepting 
it. 

Professor Hopkinson decided to test the truth of this 
assumption for himself, and he carried out a complete series 
of tests, using the reflecting type of indicator already men- 
tioned. He found that " the difference between indicated 
horse-power and brake horse-power is rather less than the 



THE GAS ENGINE. 



143 



A.L 




Spring 250 




Spring 250 



Fig. 46. — Indicator Diagrams taken from 500 B.H.P. " Oechelhauser " 
Gas Engine. Cylinder 26 -^" bore. Front stroke 37^". Back stroke 
37V'. When calculating I.H.P. from these cards it must be remem- 
bered that the engine is a two cycle one. Compare with Fig. 50. 

horse-power at no load under the same conditions of lubri- 
cation, mainly because of the difference in the power 



Fig. 47.— B.H.P. tests. When the 
mechanical efficiency of an engine 
has to be measured, both I.H.P. 
and B.H.P. must be found. The 
B.H.P. when measured by a fric- 
tion brake as shown in this dia- 



W-S 



x c x r.p. 



_ 33000 

where c is circumference of pulley 
in feet, and W and S are measured 
in pounds. 




144 THE INTERNAL COMBUSTION ENGINE 

absorbed in pumping. In the particular engine tested, 
the error from this cause in obtaining the indicated power 
would amount to about 5 per cent. The friction is sub- 
stantially constant from no load to full load, provided 
that the temperature of the cylinder walls is kept the same, 
"but the influence of temperature is very great." He found 
the mechanical losses in a 41 h.p. engine to be as follows : — 
Suction . . . . .34 per cent, of i.h.p. 

Piston friction. . .61 ,, ,, 

Other friction (valve lifting, etc.) 2-7 ,, ,, 

Total .... 12-2 
The various efficiency figures for this engine were : — 
Thermal efficiency . . 33 J to 37 per cent., according 

to strength of mixture. 
Mechanical efficiency for 

medium charge . 85 to 90 per cent., according 

to jacket temperature. 
' * Air-standard" efficiency. 52-2 per cent, (on compres- 
sion ratio of 6 37). 
Efficiency relative to " air- 
standard" . . . 0-64 to 0-71. 

70. Analysis of Motion of Indicator Piston. — The piston used in the 
indicator instrument cannot be absolutely weightless whatever 
improvement may be made in reducing the weights of the moving 
levers (either by adopting lighter scantlings or by using a beam of 
light). Let the pressure acting on the base of the piston of mass M at 
any time to be p, also let piston area = a and the motion of the piston 
"be S 2 inches for each pound per sq. inch of pressure acting upon it. 
Then the forces acting on the piston when at a point x above its 
lowest position are : — 
upwards pXa 

downwards -~ -a + M'-jr 2 - 

-, r d 2 x a 
therefore M—jT^- + -^-'x = p.a. 

d 2 x , a pa ,,, 

-d^ + m x= ^ (1) 

Integrate this. The Particular Integral is 

1 pa _ S 2 M 1 pa 

x= T~ a~ ' ~M = ~a~ SoM ' ~W 



THE GAS ENGINE 145 

S 2 M pa 

so that x= X -*>- = V- &2 

a M r 

and the Complementary Function is 

So that the Complete Integral is 

x- A sin ,\/ s « M .t + B. cos A /^. t + p.S 2 . 

Now when t=o,x=o 
therefore B = — pS 2 

dv / a - . . / a 7-, 4 / a . „ / cs 

"17 = A/ • -^ cos A / - — - . t — B A / -=- — . sin A/ — — — . t 

dt V s 2 M v S 2 M V £ 2 M v £ 2 M 

eta 
and when t=o,—r. = o 
dt 

therefore A = o 

Substitute these values of A and B and 

x— pSo ) 1 — cos A / . £ r . This means that 

(- V £ 2 ikf > 

the piston rises to a height p>S 2 and then oscillates about that 

position with a frequency equal to ^— a/ ■ . 

All this assumes, however, that p is a constant or that it increases 
with such rapidity that it assumes its final value before the indi- 
cator piston has had time to move. It would have been more 

accurate to assume p to rise from zero to its final value in, say, — th 

n 

part of a second and to consider what happens during this interval. 
To do this put p = a v t where a 1 has the constant value given by the 

equation : — final value of pressure = — -. 



Equation 


(1) 


now 


■ becomes 












d 2 x a 

~dW + Km' 


x = 


a 
M 


and the Particular 


Integral 










X- 


_S 2 M/ l S,M 
a \ a 


D*X 


-i 








S 2 M a n 
'~a~' M "* 


= S 2 Cl^t' 



(2) 



aa-L 
~M 



Therefore the Complete Integral would be 

x = A sin \/JL- .t + B cos \ / -^— . t + 5f a a^. 

x S 2 M * S 2 M 

And since x = o when t = o 

therefore B = o 



146 THE INTERNAL COMBUSTION ENGINE 

Again Tr^/^- AoOS s/m t + 8Vh 

but when x — o t = o 

so that o = A / . A + £2% 



and 



» n 



This gives us a; = S.aJ—S,^ \/^l sin . /^U- * 






Xow fi^a^ is height to which piston would rise under the slow 
static pressure — call it h so that S 2 a t t = h, and let / be the frequency 
of the free vibration of the indicator piston. Then 

t 1 4 / a O 4 1 / a 

f = ^r~ \ / or 277/ = A / 

<S 2 a,. / a~ 

so that • r = ft -2W" Sm V^S'' 

or x = 7n 1 — ^—r- sin 27r/£ , v (3) 

I 2nft ' i 

This means a fractional lag of ^—7. as a maximum, but for any 

& 277/^ 

particular case it can be calculated thus. We may put / as 300, which 
about represents the best modern practice using an instrument of 
the Hopkinson type. 
Then 

X = 7r l 1 -I890^ sin1890 ^" 

It will be useful to compute a few values for this for cases in which 
the value of t is much shorter than the periodic time of the instrument. 
When this is so sin 1890£ can be written with sufficient accuracy as 

(1890*) 3 
(18900- —^ ± 

( (18900 2 ) 
or x=hYV — 1 + « — ,- 

(18900 2 
= h .- — ^- J -= 600,000^ 2 . 
6 

The relation between x and t in these early stages is therefore para- 
bolic. The time t starts, so to speak, first, but x soon increases and 
gradually catches up. 



THE GAS ENGINE 



147 



Thus for 


* — 100000 SGC. 




x 600,000 „ „„„„„ 
h - (100,000)- 000006 


For 


t = Toooo SGC. 




-r- = 0-006 
h 


for 


t = Tooo SGC. 




— = 0-6 
h 



but for this valuG of t our approximation no longGr holds. For 
t = yJgg the calculation should procGGd thus 

x /, 1000 . , oftA \ 

— = [1 — sin 1-890] 

h \ 1890 J 



= 1- 



.890 
1 

1 

T89 



sin 108° 
X 0-95 =1—0-50 



.-. — = 0-50 
h 

showing that thG instrumGnt is picking up. Evidently theroforo it 
will not do to use an instrument for recording an explosion occurring 
in tooo sec. unless its own frequency exceeds 300. 

The following table shows a series of values, and in Fig. 47 they 
are shown plotted. 



£secs. 


1890* 


sin 1890* 


( 1 .sin 18900 
M890* l 


X 

~h~ 


Tooooo 











000006 


Toooo 


— ■ 


— 


— 


0006 


2000 


— 


— 


— 


015 


i 


1-89 


0-95 


0-50 


0-50 


i 

500 


3-78 


-0-60 


016 


116 


Too 


4-72 


-100 


-0-21 


1-21 


*io 


7-56 


0-96 


013 


0-87 


Toi) 


18-9 








100 



Whenever t is a multiple of ^ the value of -r-will be 1 00. The 

above curve does not of course take account of the frictional forces 
which prevent the indicator piston continuing to vibrate indefinitely. 
Students are recommended to work the problem out, introducing 
into equation (2) a term representing the frictional force. The result 
will be to multiply the oscillatory term by a factor of the type 
f— 'it which, when the student has plotted the resulting curves, will 
show that the straight line is soon followed once the curve comes 
up to and crosses it. From the curve in Fig. 48 it is clear that 



148 THE INTERNAL COMBUSTION ENGINE 



for recording an explosion occurring in T o£oo sec « tms indicate .■ 
with its gig period would be inadequate. The piston would scarcely 
have moved. For an explosion occupying ten times as long, i.e. T ^^ 
•sec, the indicator would still be lagging a long way behind. For a 
? i^ sec. explosion the actual maximum pressure would be very fairly 
represented, but not the shape of the explosion wave. In fact for 
useful readings the instrument should not be used for any sharper 
explosion than ^^ sec. For an ordinary gas engine explosion occurring 
in _ j_ sec. the instrument would be quite satisfactory. 

71. Heat Balance-sheets. — A heat balance-sheet as applied 




Line showing actual 
rise of Pressure . 



Line showing record made 
by undamped Indicator. 



-yen Time in Seconds 



Fig. 48. — Diagram illustrating the way in which an undamped Indicator 
would follow a rapid explosion. Period of Indicator, a£ s sec. 

to a gas engine is a statement of the way in which the total 
amount of heat passed into the engine is employed. In the 



THE GAS ENGINE 



149 



early days of gas engine work it was easy to remember that 

roughly — 

Heat passed to water jacket . . 40 per cent. 

Heat left in exhaust gases . .40 „ 

Heat converted into work . .20 „ 

100 

Such balance-sheets have, however, lately become a good 
deal more complicated. First, there is the difficulty of 
knowing how much heat the exhaust gases really carry 
away, the specific heat not being accurately known ; and 
then there is the difficulty that the exhaust gases on their 
way out usually part with some of their heat to the water 
jacket. This leads to part of the loss being counted twice 
over so that the total instead of coming out as 100 per cent, 
often comes out as 101 per cent, or 102 per cent. 

In the experiments made by the Institution of Civil 
Engineers Committee already referred to the full-load 
heat balance-sheet was given as : — 



Designation of Engine. 


L. 


E. 


X. 


Exhaust waste 

Jacket waste 

Radiation 

B.H.P 


35-3 
23-5 

7-6 
26-7 


400 
29-3 
100 
28-3 


39-5 

250 

7-3 

29-8 


Total 


931 


107-6 


101-6 



In these experiments the exhaust waste was measured 
by passing the exhaust gases into a water-jet calorimeter. 
Jacket waste was measured as the product of quantity of 
cooling water passed and rise of temperature. Radiation 
includes engine friction as well as radiation proper. B.H.P. 
was measured by a rope brake. 

Engine L shows a deficit in the total, so that there must 
have been something wrong in the experiments. Mr. 
Dugald Clerk in his paper * before the Institution of Civil 
* Head February 2G, 1907. 



150 THE INTERNAL COMBUSTION ENGINE 

Engineers, " On the Limits of Thermal Efficiency in Internal 
Combustion Motors," endeavoured to correct this measure- 
ment from several different possible points of view. He also 
extended the same treatment to tests B and X in order 
to get the true balance-sheet, and putting in i.h.p. instead 
of b.h.p. (the Committee's records were complete enough 
to permit of this), he found : — 



Designation of Engine. 


L. 


R. 


X. 


Exhaust waste 

Jacket waste and radiation 
I.H.P 


410 

27-2 
31-8 


371 
29-6 
33-3 


39-9 
25-4 
34-7 


Total 


1000 


1000 


1000 



Mr. Clerk then points out that the 27-2 per cent, of jacket 
waste and radiation for test L is obviously too low, and that 
heat appears to have been lost in some way. He therefore 
took the total of the exhaust waste and jacket waste and 
radiation items, i.e. 682 per cent, and attributed 34-1 per 
cent, to each, so making the balance sheet into : — 



Designation of Engine. 


L. 


R. 


X 


Exhaust waste 

Jacket waste and radiation . 
I.H.P 


341 
341 
31-8 


371 
29-6 
33-3 


39-9 
25-4 
34-7 


Total 


1000 


100-0 


1000 



Mr. Clerk considered this balance-sheet probably re- 
presented the distribution of heat in the engines more 
accurately than either of the others. 

72. These various attempts at a heat balance-sheet have 
been given in order to show how very difficult it is 
to obtain a really accurate statement. The exhaust wastes 
originally given for L, R and X were 35-3 per cent., 40 per 



THE GAS ENGINE 



151 



cent, and 39 5 per cent., and have now become 34 1 per 
cent., 37 1 per cent, and 39 9 per cent. 

But the matter does not end even here as Mr. Clerk 




brought into use the values found by him for the specific 
heat — values which showed a marked increase with rise of 
specific heat — and used them in some separate experiments 



152 THE INTERNAL COMBUSTION ENGINE 



of his own with the engine X used by the Committee, 
then found that the balance-sheet became : — 
Heat-flow during explosion and 

expansion Hfl 
Heat contained in gases at end of 
expansion .... 
I.H.P 



Compare this with the balance-sheet given on p. 
based on the Committee's experiments : — 



He 



161 


per 


cent. 


493 




>? 


34-6 




j? 


1000 





150 



Committee's 
Trials. 


Mr. D. C's. 
Trials. 


Heat-flow during explosion and expansion 25-4 
Heat contained in gases at end of expansion 39-9 
I.H.P 34-7 


16-1 

49-3 
34-6 


Total 100-0 


1000 



The discrepancies shown here are indeed serious. Mr. 
Clerk's comment on them is as follows : " The indicated 
work is practically the same in both trials and the sum of 
the other two items is the same also, but the distribution is 
different. Less heat flows through the cylinder-walls as 
determined by the author's (Mr. Clerk's) new method, and 
the exhaust gases contain more heat than the Committee's 
calorimeter trials show. The ordinary trials show 9-3 per 
cent, too much heat as passing through the cylinder- walls, and 
practically the same amount too little appears in the exhaust 
calorimeter. That is, 18-8 per cent, of the total heat 
remaining in the hot gases at the end of the expansion 
passes into the cylinder water-jacket during the flow through 
the exhaust valve upon the first opening and while the 
piston is making its exhaust stroke. This seems to be a 
quite reasonable portion of the total heat, such a portion 
as experience would lead one to expect. These new diagram 
trials afford, in the author's (Mr. Clerk's) view, a more 
accurate heat-distribution balance-sheet than has yet been 



THE GAS ENGINE 15a 

obtained in any engine, from which can be deduced the 
ideal efficiency of the working fluid. Adding together 
Heat contained in gases at end of expansion 49-3 
I.H.P 34-6 



83-9 



Then —— =041. That is, if this balance-sheet be correct 

oo'v 

and the heat loss be assumed as entirely incurred at the 
beginning of the stroke, then the maximum efficiency of 
the actual working fluid for the compression and expan- 
sion is 41 per cent, of the total heat supplied." The 
earlier part of this quotation is the subject of our present 
discussion, the latter part is dealt with elsewhere. 

Even with such very considerable discrepancies in the 
heat balance-sheets as those discussed above, the student 
will none the less remark that the heat utilized has now 
grown from about 20 per cent, to over 30 per cent. This 
all-important improvement has occurred therefore in spite 
of the many uncertainties as to how the lost heat divided 
itself up. It is indeed one of the fortunate features cf gas 
engine manufacture that improvements do not have to 
attend the settlement of the many intricate problems 
with which gas engine operation is bound up, but proceed 
by the trial and error of experiment with such guidance as 
theoretical considerations have been able to afford. The 
great want which has in the past caused so much theoretical 
difficulty has been accurate knowledge of the values of the 
specific heats of the working fluids. 

73. Testing of Gas Engines. — The very scientific attitude 
adopted by the Germans to engineering work has led them 
to lay down precise rules for the testing of gas plant. These 
rules have been drawn up by the German Associations of 
Engineers, engineering firms, and large gas engine builders 
and published in detail in the Zeitschrift des Vereines 
Deutscher Ingenieure. Their object is to standardize pro- 
cedure throughout the country and so render tests properly 
comparable. The more important rules are that — 



154 THE INTERNAL COMBUSTION ENGINE 

1. Fuel consumption tests of gas producers are to last 

eight hours without interruption. 

2. Fuel consumption tests on engines are to last one hour 

at high loads and a lesser time at low loads. 

3. To determine that the engine has reached a steady 

state the temperature of the cooling water will be 
measured from time to time. 

4. Mechanical efficiency tests must be taken at constant 

load and at least ten series of diagrams taken. 

5. Temperatures to be Centigrade. One h. p. -hour to be 

taken as 632 calories. 
74. Engine Tests. — The result of a test on a 200 h.p. 
engine and suction plant has been published by Mr. Mathot * 




Fig. 50. — Typical Indicator Card from a Four Cycle Engine. Compare 

with Fig. 46. 



and the more important figures are here reproduced. The 
engine was of the four-cycle double-acting type and was 
tested at the works of the well-known firm of Gasmotoren 
Fabrik, Deutz-Cologne. 

Piston diam. . . . . . 21 J in. 

stroke ..... 27 T 9 g in. 

„ rods diam. — 
Front . 
Rear . . . ... 



4f 



m. 



1 6 



in. 



* I.M.E., 1905. 



THE GAS ENGINE 
Full Load Tests. 



155 



1904. 



March 14. 



March 15. 



Average r.p.m. 

B.HP . 

Duration of test, hours 

Average temperature of water after 
cooling piston 

Average temperature of water after 
cooling cylinder and valve seats . 

Water consumption for cooling piston, 
gallons /hour 

Water consumption per hour in vapor- 
izer (anthracite fuel), galls, /hour . 

Water consumption per hour in scrub- 
bers, galls, /hour 

Average temperature of gas at outlet 
of generator 

Average temperature of gas at outlet 
of scrubbers 

Gross fuel consumption per b.h.p.hour 

■Corresponding thermal efficiency . 



151-29 

214-22 

3 

117-5° F. 

135° F. 

39 



15020 

222-83 

10 



0-727 lb.* 
19 per cent. 



14-2 
318 

558° F. 

62-5° F. 
0-720 lb. 
24-4per cent. 



Other interesting figures are — 

Water consumption in galls, per b.h.p.-hour — 

1. For cooling cylinder, stuffing boxes, valve 

seats and jackets. .... 465 

2. For cooling piston and piston rods . .1-75 

3. For vaporizer ..... 00655 

4. For washing the gas in the scrubbers . 1-42 
Also : — 

Water converted into steam 
per lb. of fuel consumed in 

generator .... 0193 galls, or 193 lb. 
In an important test carried out by Mr. J. T. Nicolson on 
a Crossley gas engine and suction producer plant, the 
calorific value of the gas was 156-5 B.T.U. as determined 
by analysis and 149 B.T.U. per cubic foot by Junker's 
calorimeter at the temperature and pressure of the calori- 
meter. The following measurements were made : — 
* Includes fourteen hours of fires banked up. 



156 THE INTERNAL COMBUSTION ENGINE 

B.H.P. =559. 

Gas per hour =29,037 cu. ft. corrected to 0° C. and 760 mm. 

Gas per b.h.p. =51-94 cu. ft. 

Heat supplied =51-94 x 1565 =8,128 B.T.U. per b.h.p.-. 

, , ., , oa • 1,980,000 1 2,546 
hour and thermal efficiency = x = = 

J 778 8,128 8,128 

319 per cent. 

Variation in engine speed when horse-power was in- 
stantaneously dropped from 600 to 50 was from 119-4 to 
121-4 r.p.m., corresponding to a total variation of If per 
cent, of mean speed. No back-firing was observed to take 
place when this was done. These tests show remarkably 
good thermal efficiency and satisfactory closeness of govern- 
ing. Engines of this size have not often been run on suction 
gas. 

A third trial is that of a 150 b.h.p. six cylinder vertical 
gas engine which was run for six hours on full load. The 
gas was taken from a pressure producer and had its calorific 
value measured every hour by a Simmance-Abady Calori- 
meter. Readings were taken every half -hour of the b.h.p. 

Average air temperature . . 72-2° F. 

Average air pressure . . 29-56" Hg. 

Cu. ft. gas used per hour . 13,000 

Average Calorific value (lower) . 128-1 B.T.U. per cu. ft. 

Engine speed . . . 325 r.p.m. 

B.H.P. = .... 151-3. 
B.T.U. consumed by engine per b.h.p. -hour = 10,590, showing 

a thermal efficiency of — ' =24-1 per cent., which was 

J 10,590x778 

up to the standard of the intended design. 

75. The Governing of Gas Engines. — The most frequent 
method of governing in this country has been by means of 
the " hit-and-miss " gear, which consists of an arrangement 
whereby a small piece of metal normally interposed between 
the valve operating level and the valve spindle is moved 
away by the governor with the result that although the 
valve level continues to rock it is unable to communicate its 
motion to the valve. This is a very simple arrangement, 



THE GAS ENGINE 157 

but on the Continent it is considered to be hardly sensitive 
enough to slight changes of speed since the valve either 
opens to its full amount or else does not open at all. To 
understand the effect produced it is only necessary to take 
the case of a steam engine in which the governor either 
closed the throttle valve altogether or else did not alter 
it at all. For very great uniformity of speed it is necessary 
to employ some governing mechanism which shall work in a 
more gradual manner. There is a further objection which 
lies against the " hit-and-miss " system in that to produce 
a reasonable measure of uniformity of angular velocity in the 
crank shaft a very heavy flywheel becomes necessary. This 
adds to the cost of the engine and diminishes its mechanical 
efficiency. 

It may in fact be said that the two merits which have 
enabled the " hit-and-miss " gear to be used as much as it is, 
are its great mechanical simplicity and its ability to keep 
constant the proportions of gas and air in the incoming 
charge, so enabling the engine always to be run on its most 
economical mixture. 

Continental makers were the first to break away from 
this system of governing, by arranging that the governor 
should produce a variable lift of the gas valve by means of 
a conical cam. As, however, the air supply was not inter- 
fered with this meant a continually changing richness of 
charge and hence a lowering of thermal efficiency. This 
lowering of efficiency would be attributable to the fact that 
there is for a given engine only one mixture which will give 
the best efficiency of explosion, and that as the richness 
changes so the time taken for the mixture to ignite will 
change, and therefore with a fixed ignition point the maxi- 
mum pressure will not come at the most advantageous part 
of the stroke. The tendency now is towards a regulation 
of both the gas and the air supplies by throttling them 
after mixture, with the consequence that less weight of 
explosive mixture is taken in and therefore a less compression 
pressure is reached. Using this process it is necessary 
to make the compression with full mixture fairly high so 
that when on lighter load there shall still be enough com- 



158 THE INTERNAL COMBUSTION ENGINE 

pression left to enable a sufficiently good thermal efficiency 
to be obtained. 

76. Flywheel Effect — The kinetic energy stored up in a 
flywheel is calculated from the following formula or one 
derived from it. 

KE.=iI.coHtAb. 

where /^moment of inertia about the axis of revolution 
and o)=angular velocity in radians per second. 
From this it follows that 

— {KE.)=I.a>. 

For a small variation in a> compared with KE. it is there- 
fore necessary that either lorw should be large. For the 
ordinary purposes of industry it is sufficient to ensure that 
the angular velocity never varies by more than ^- th to 3^th 
part above or below the mean speed. For the driving of 
continuous current generators only half the above variations 
are permissible, whilst for the driving of alternators in parallel 
the requirements are far more stringent, involving a per- 
missible variation of but T \ ^ th or even in some cases 2lro th 
part above or below the mean. Mr. Mathot,* the well- 
known gas engine engineer, has suggested the following 
formula for use in calculating the dimensions which should 
be given to flywheels of different types of engines : — 

D 2 .a.n 3 
where 

P=weight of rim (without arms or boss), in tons. 
Z)=diameter to centre of gravity of rim, in inches. 
a = degree of cyclic irregularity permissible. 
n =revolutions per minute. 
2VT=b.h.p. 

^^coefficient determined as below. 
For Otto cycle engines, single cylinder, single 

acting & = 44,000 

For Otto cycle engines, two opposite cylinders, 

single acting, or one cylinder double acting . £=28,000 
* I.M.E., 1905. 



THE GAS ENGINE 



159 




160 THE INTERNAL COMBUSTION ENGINE 

For two cylinders, single acting, with cranks 

set at 90° h =25,000 

For two twin cylinders, single acting . . k =21,000 

For four twin cylinders opposite, or for two 

tandem cylinders, double acting . . . h— 7,000 

Note. — Total weight of flywheel may be put as 1-4 P. 
77. The most usual way to speak of cyclic irregularity 
is that above described. It amounts to defining cyclic 
irregularity as the fraction by which the instantaneous 
angular velocity exceeds or falls below its mean value in 
any one complete cycle. There is, however, another way 
of considering the matter. Thus Mr. L. Schiiler in a paper 
dealing with the driving by gas engines of alternators 
•operated in parallel remarks : " The speed of the machines 
should be as uniform as possible and should in any case be 
such that the amplitude of the angular oscillation does not 
•exceed two electrical degrees." An electric degree means 
of course -g^o tn P art °f the angular distance, the passage 
of which by the armature corresponds to one electrical 
cycle. It becomes therefore a matter of interest to see how 
the one form of computation can be turned into the other. 
A good deal depends naturally on the rate at which the speed 
variation rises and falls, but for a sufficiently close approxi- 
mation it may be taken as a sine or cosine curve. Then 
the angular velocity w may be written as equal to 

a+b cos c6. 

Where is the angle the crank has moved through, a is 
the mean value of the angular velocity, and b is its maximum 
variation from the mean, so that the speed oscillates between 
(a -\-b) and (a — 6). If c be unity this oscillation occurs once 
in a revolution, but if c=2 then it occurs twice, and so on. 
This may be written 

-a-\-b cos cO 



or dt — 



dt 

dO 



a-\-b cos cO 
integrate and 



THE GAS ENGINE 161 

, cO 

2 v a — b tan-r 

c.t —A + — r=^^-tan~ 1 = 

va 2 —b 2 ^a+b 

where A is some constant. 

Put 0=0 when 2=0 and therefore A =0 

, cO 

2 va — 6 tan — 

so that t— — tan" 1 — - •• •• (1) 

cva 2 - 6 2 v'a+6 

Now - is the cyclic irregularity and may be given a 
a 

name — call it m. 

, cO 

vl — mtan-^ 

— i 

therefore t — , o tan , — 

ac v 1 — m 2 v 1 -fm 

cO 



vl — m tan 



2 ac£ v 1 — m 2 

=tan 



vl+m 2 

a 2 1 ry/\-L. m ac^l — m 2 -> 
or y =— tan ) _ J = ,tan ( 

-m 2 j 



5— 



(2) 



Now as m is always small compared to unity vl-fra may 
be written as (1 + Jm) and m 2 be neglected ; 

therefore = — tan - 1 i ( 1 +m) tan — f • 

c L 2 ) 

Now were m really zero this equation would give to a 

value equal to 

2 , _ x , act 
= — tan x tan — —at. 

c 2 

Call this value . Really it means the position at which 
the crank would be, were the angular velocity strictly uniform . 

We may therefore write = —tan -1 J ( 1 +m) tan ° [ (3) 

which is the solution. 

If, for example, c=l and in — 

* 200 



162 THE INTERNAL COMBUSTION ENGINE 

then 6=2 tan" 1 -1005 tan-5- 1 . 

1 2 ] 

From this we see that when 6 = 120°, becomes 
2 tan" 1 {1-005 tan 60°} 

=2 tan" 1 (1-005 xl-73205) 

=2 tan" 1 1-74071 

=2 x 60-12° 

= 12024° 
or that the crank would be nearly J degree ahead of its 
true position. 

It is useful to get an expression for this deviation directly. 
From (3) 

0— G^Atan- 1 Ux+m) tan^?|— O . 

Find the maximum value of — 6 Q by differentiating and 
equating to zero. Then 

(1 + m ) = 1 1 +(1 +m) 2 tan 2 — °1 cos 2 ^ 

rO c6 

= cos 2 — ! ) +(l+m) 2 . sin 2 — 
2 ^ T ' 2 

rO c6 

l=2sin 2 — °+™sin 2 — ° 

2 2 

sin 2 — ° = 



2 -{-771 

2 1+ra 
or tan— =1 — J??? approximately, 

so that the maximum value of — is found from the 
expressions 



and 



o = Itaa-^l— Jm). 

c 

Therefore the maximum deviation 



THE GAS ENGINE 163 

=— {tan-^l +im)— tan-^l— Jm) \ 

r. ) 



C 

2 , _ 1 4m 
— tan 



c 8 — m* 

If m be so small that m 2 can be neglected — as it practically 
is — this reduces to 

Q YY) 

maximum deviation = -tan -1 - . . . . (4) 

o 2 

If for example c = l and m = — , this equation gives the 
maximum deviation 

=2 tan -1 — =2x1-2=2-4 degrees. 
50 5 

When m is much smaller than — , say equation (4) can be 

approximately written : — 

_ x — = — . So that with c = 1 and m = — the maximum 
c 2 ■ c 200 

deviation would be — radian or 0-28 degree. 
200 & 

78. Equation (2) shows how the value of 6 can be cal- 
culated for any position of the ideal crank, and the deviation 
may have its most important effect electrically even when 
it has not itself its largest numerical value. For that 
reason it is desirable to have some means of calculating 
it easily. In cases in which it is only desired to find the 
maximum deviation to some approximate degree of ac- 
curacy it is sufficient to take a mean value of the excess 
angular velocity and multiply it by the time during which 

it operates. Thus if as before c = l and m= — , calling the 

angular velocity w, the average excess of angular velocity 

2 (o (0 

" 7T ' 200 "" 100- 

and this operates through 180° or for a time equal to , 



164 THE INTERNAL COMBUSTION ENGINE 

so that the angular motion gained 



O) IT 1 

X 



IOOtt w 100 

and radian =0-57 deg., which, divided between the 

100 

two ends of the period, gives a maximum deviation of 0*28 
deg., which agrees with the 0*28 deg. previously found ; but 
for larger values of m, this method would be less accurate. 

For these values of c and m it may be said that the maxi- 
mum deviation is about J of a degree. If the alternator 
has six pairs of poles giving six electrical cycles during one 
mechanical one this deviation could also be called J x6 or 
one and a half electrical degrees, which corresponds very 
well with Mr. Schuler's result. 

79. Balancing. — The problem of balancing the parts of a 
gas engine and providing for uniformity of torque as far 
as possible does not differ in principle from the correspond- 
ing problem in the case of the steam engine, and the author 
does not propose therefore to devote a great deal of space 
to this subject. The student should refer to what has been 
written on the subject of balancing by Professor Perry and 
Professor Dalby, both of whom have made a special study 
of the matter. It will, however, be advisable to give here 
a brief account of the general principles involved, leaving 
the application to be made to each and every problem as 
it presents itself. For it must be remembered that although 
the problem is often surrounded by complications which 
lead to the mathematical work looking difficult and involved, 
there is really no special difficulty about it at all, but merely 
a necessity that the fundamental principles should be 
rightly applied and that the algebraic or arithmetical work 
should be carried through without mistakes. 

The simplest kind of balancing is that in which a flywheel 
is light on one side and requires a weight ( W) fastened to the 
other side in order to prevent any jumping or vibration 
when the wheel rotates. This does not of necessity mean 
that an equal weight must be added to the other side, 
because it does not follow that it will be possible to place 



THE GAS ENGINE 165 

the balance weight at the same distance from the centre 

W 

of the shaft, and the centrifugal force being equal to w 2 .r.— 

(where w=angular velocity in radians per second and r = 
distance in feet from the centre of the shaft) it is evident 
that the product of the W and the r in the balance weight 
must come out to a certain amount. If therefore the r is 
very small then W must be proportionately greater, and 
inasmuch as the balance weight is often bolted in between 
the spokes it is clear that r will usually be less than the 
radius of the rim of the wheel. This is the simplest kind of 
balancing. The most complicated kind occurs in the 
motion of a rod, like a connecting-rod, in which one end 
reciprocates to and fro in a straight line and the other 
end follows a circular path, with the result that intermediate 
parts of the rod follow a complicated curve and one not easy 
to treat. In such a case as this it is customary to obtain 
an approximate solution by assuming that a certain part of 
the rod is massed at the crosshead and the rest at the crank- 
pin, and it is not unusual to make this division of the rod in 
inverse proportion to the distance of the centre of gravity 
from either end. This is only an approximation unless it 
happens that the rod is so made (which it usually is not) that 
if hung up from the big and little ends in turn it will swing, 
pendulum wise, with the same number of swing-swangs per 
minute. When a number of rotating masses (real or 
assumed) have to be balanced it is useful, following Dalby's 
method, to consider the plane perpendicular to the shaft in 
which one or more of them lie to be rotating at the same 
speed as the shaft and to draw out on this plane the force 
diagram. 

80. The connecting rod comes into the problem in yet 
another way. If the crank-pin rotated uniformly and the 
connecting rod were exceedingly long the motion of the 
piston would be a Simple Harmonic Motion (usually written 
S.H.M.) and the displacement of the piston from the middle 
of its stroke would be equal to r. cos where r =radius of 
crank-pin circle and is the angle between the crank and 
the line of dead centres. But the connecting rod in actual 



166 THE INTERNAL COMBUSTION ENGINE 

engines is usually quite short, never more than ten 
times the length of the crank arm and usually much less. 
This produces a complicated motion of the piston, and it 
will be useful to calculate exactly what it is. Let P be 
the crank-pin andyl the piston which, in the position shown, 
is at a distance AB from the beginning of its stroke. The 
angles and (p are as shown in the diagram. OP is r and 
AP, I. AB will be written as x. Now it is clear that 

r cos d-\-l cos (p-\-x—BO 
and BO=l+r 

so that r cos 6-\-l cos <fi-\-x=l-\-r (1) 

Also we have r sin 6=1 sin <p (2) 

It is necessary to combine these two equations so as to 
find x in terms of known quantities. 



Fig. 52. — Motion of Crank-pin and Connecting Rod. 

From (1) 

x= l-\-r — r cos — I cos (j) 
= 1(1— coB(p)+r{l— cos 6) 



Also from (2) 



T 

sin <p= — sin 6 





/ r 2 
and cos <p= V 1 — sin 2 



(3) 



therefore x=l(\—x/ 1— ^-sin 2 fl) +r(l— cos 0) 

and this gives the value of x for any value of 0. 

Previously we have spoken of the distance of the piston 
from the mid point of its stroke rather than from either 
end, and it is useful to follow the same procedure here — 



THE GAS ENGINE 167 

call the displacement of the piston from mid stroke y — then 

x-\-y—r 

or y =r — x 

so that y—r cos — 1( 1 — a/ l — JL sin 2 ) 

furthermore is a function of the time, and as uniformity of 
rotation is assumed it will be directly proportional to the 
time. Put - therefore 0=cot 



so that y —r cos cot — l(l — a/ 1 -^sin 2 cot) • • •• W 

Since, however ( — J is a small amount in all engines 
an approximation to the above may be written as 
y—r cos cot — 1( 1 — 1 + i — sin 2 cot J 

r 2 . ' 
=r cos cot — — sin- cot 
21 

or y=r cos cot -(1 — cos 2wt) (5) 

This very interesting result shows that the position of 
the piston can be stated as the sum of two S.H.M.'s one 

of which corresponds to an infinitely long connecting rod 
and the other to a S.H.M. of twice the periodicity and of an 
amplitude depending on the ratio of r to I. The motion 
in fact is analogous to that of the air set into vibration 
by an organ pipe which in addition to giving its fundamental 
note gives also a weak first harmonic. Although this first 
harmonic is weak in its effect on the displacement of the 
piston it is considerably more potent when velocities 
and accelerations have to be taken into account, as will 
presently appear. 
Since from (5) 



.2 



y—r cos cot — (1 — cos 2wt) 

dy . , r 2 (o . 

; = — cor sin cot — sin 2u>t 
dt 2/ 



168 THE INTERNAL COMBUSTION ENGINE 

= — wrl sin wt-\- — sin 2cotj .. .. (6) 

d^"ii f v \ 

and — — = — t» 2 r( cos wt-\- cos 2cot) .. .. (7) 

dt 2 V I J V } 

It is important to note that expression (6) which gives 
the velocity of the piston at any point has the multi- 

T 

plier — in front of the harmonic term, and that expression 
(7) which gives the acceleration and therefore measures all 

T 

the inertia forces has the multiplier — . It follows therefore 

V 

that the three multipliers in the harmonic term for displace- 
ment, velocity and acceleration run thus, — , — and — , 

showing that a ratio of — which will produce a 5 per cent. 

V 

difference in the position of the piston will bring about a 
10 per cent, change in the velocity and about 20 per cent, in 
the acceleration. It will now be realized that, when forces 
are being nicely balanced, the growing importance of the 
harmonic term must be carefully allowed for. 

It is often useful to bear in mind a simple rule for the 
value of the acceleration at the ends of the stroke, i.e. 
when atf=0° or 180°. From formula (7) it will be seen that 
this leads to 

^| either =— »vfl'+— V or = — »v(— 1+y ) 

= = F »V(l±y) 

a very simple approximate rule, i.e. that the acceleration 
at the end of the stroke is more or less than the S.H.M. 

T 

value by the fraction — of that value. 

81. Connecting Rod Effect. — This is best illustrated 
by a geometrical construction due to Mr. J, Harrison, 
MXC.E, 



THE GAS ENGINE 



169 



The construction is as follows : — 

OB is the crank and AB the connecting rod of which G 
is the centre of gravity. 

OQ and SH are perpendicular to AO. 
Ha is perpendicular to AB. 
SQ and Gg are parallel to AO. 
GU=k 2 /AG where &=radius of gyration. 
UX is parallel to Ba. 




A a N 

Fig. 53. — Resultant of all the Accelerating Forces on a Connecting Rod. 

Then TXN parallel to gO is the line of action of the 
resultant of all the forces acting on the rod and its value 
in mq 2 .XN where m=mass of rod and #=angular velocity 
of crank-pin. The proof of this construction is given in 
Professor Perry's Steam Engine, p. 549, and may there be 
referred to by those interested. 

This diagram enables the direction and amount of the 
inertia forces due to the connecting rod to be calculated 
for each position of the crank. 

If k 2 happens to be equal to the product of AG and GB 
then GU—k 2 /AG—GB so that the point U would coincide 
with B and the resultant force would pass through and 
hence there would be no " whipping effect " of the rod. 
One often sees connecting rods produced beyond the crank- 
pin with the ob j ect of bringing about this relationship . When 
it is accurately obtained it will be found that the period of 
swing of the rod about either big or little end will be the 
same. 



PROBLEMS. 

1. Describe a gas engine, and how it uses the Otto cycle 
of operations, Sketch the cylinder, showing piston, water- 



170 THE INTERNAL COMBUSTION ENGINE 

jacket, valves, shape of clearance space, and shape of 
exhaust outside the cylinder. We do not want sketches 
of any of the other parts of the engine, although they are 
mentioned in the description. Draw the usual sort of 
diagram to scale. (B. of E., 1899.) 

2. Describe the cycle of operations in a common gas engine. 
Sketch and describe the construction and action of the 
mechanism by which the speed of the engine is controlled. 
What would be the i.h.p. of such an engine whose piston 
is 12 in. diameter, its crank is 8 in. long, the engine makes 
160 revolutions or 80 cycles per minute, and 30 per cent, 
of the possible explosions are omitted ? The mean area 
of all the diagrams on a card taken with a 120 spring in the 
indicator as measured by the planimeter is 2-62 sq. in. ; 
length of diagram parallel to atmospheric line 4-03 in. 

Ans. 19-95 h.p. (B. of E., 1899.) 

3. Describe, with sketches, either a gas or oil engine, and 
show by a diagram how it uses the Otto cycle of operations. 
Sketch the cylinder, showing piston, water-jacket, valves, 
shape of clearance space, and how the exhaust is provided 
for. (B. of E., 1899.) 

4. What is meant by " scavenging " in relation to gas 
engines ? How is it done, and how (or why) does it affect 
the efficiency ? (Mech. Sc. Tripos, Part I, 1898.) 

5. Sketch in section a gas engine cylinder, showing the 
valves and piston. (B. of E., 1900.) 

6. Sketch a section through the gas valve of a gas engine, 
showing the hit-and-miss mechanism operated by the 
governor. (B. of E., 1907.) 

7. The mean effective pressure on the piston, both in the 
forward and back strokes, is 62 lb. per square inch ; cylinder 
18 in. diameter; crank, 18 in. long. What is the work 
done in one revolution ? Ans. 94,660 ft.-lb. (B. of E., 1906.) 

8. Sketch a gas engine indicator diagram. 

How is it used in finding the indicated horse-power ? 
State clearly what information is necessary. 
Why must we know the number of explosions per 
minute rather than the number of revolutions ? 

(B. of E„ 1900.) 



THE GAS ENGINE 171 

9. Describe, with sketches, how lubrication of the various 
parts of an engine (not encased) is now usually performed. 

(B. of E., 1902.) 

10. Assuming that an engine works against a constant 
resistance and that the energy of the moving parts of the 
engine can, at any instant, be represented by the expression 
\(M -\-mk 2 )V 2 . where V is the crank-pin velocity and k is 
the ratio between the instantaneous velocities of the piston 
and the crank-pin, show how the variations of crank-pin 
velocity during a revolution may be deduced from the 
crank-effort diagram. 

(Mech. Sc. Tripos, Part II, 1898.) 

11. A flywheel has moment of inertia / ; two wheels, one 
on each side of the first, have moments of inertia i each. 
Each length of shaft has the same stiffness s. One of the 
smaller wheels has applied to it a varying torque a sin 2irjt, 
and the other a cos 2irft. Neglect the mass of the shaft. 
What value of / will cause fracture of the shaft ? Why is 
this not exact as an illustration of what occurs between two 
cranks of a steam or gas engine and a flywheel midway be- 
tween them ? (B. of E., 1907.) 

12. Why do we regulate an engine with both a flywheel 
and a governor ? Explain clearly how each effects the 
regulation. (B. of E., 1900.) 

13. Two engines with the same centre line on opposite 
sides of a crank shaft ; same moving masses ; cranks 
exactly opposite, so that there is exact balance of horizontal 
inertia forces ; what may be done to the connecting rods 
to make perfect inertia balance ? Prove your statement. 
Is the engine perfectly balanced now ? . 

(B. of E., 1901.) 

14. Crank 1 ft., connecting rod 4 5 ft. ; what are the ac- 
celerations at the ends and some other point in the stroke, 
if the engine makes 200 revolutions per minute ? The 
piston and rod and crosshead are 420 lb. ; draw a diagram 
to show the force in pounds required to produce the motion. 
State the scale clearly. (B. of E. 5 1906.) 

15. A piston and rod and crosshead weigh 330 lb. At a 
certain instant, when the resultant total force due to steam 



172 THE INTERNAL COMBUSTION ENGINE 

pressure is 3 tons, the piston has an acceleration of 370 ft. 
per second in the same direction. What is the actual force 
acting at the crosshead ? (B. of E., 1902.) 

16. Give an account of the different methods used for 
governing gas engines, stating the advantages and dis- 
advantages of each. (Mech. Sc. Tripos, Part I, 1904.) 

17. What are the chief sources of loss in the plant con- 
sisting of boiler, reciprocating engine and condenser ? 
What is the greatest proportion of the heat given to an 
engine that can theoretically be turned into work, and 
under what conditions can this maximum be reached ? 

Show in what respect gas engines, oil engines and steam 
turbines approach more nearly to these conditions than the 
ordinary reciprocating engine. 

(Old Regulations Cambridge B.A. Degree, 1904.) 



CHAPTER VI 

The Gas Producer 

Theory — Typical Suction and Pressure Producers — Tests — 
Costs — Use of Gas Producer for Marine Purposes — 
Appendix containing Description of Mode of Operation 
of Suction Gas Plant. 

82. Producer Gas. Theory. — In a steam boiler the energy 
stored up in the coal is liberated by combustion in an atmo- 
sphere containing oxygen. In other words, heat is liberated 
by the combination of the carbon with oxygen first to 
form CO, and then, if enough air be present to add a further 
atom of oxygen to the molecule, to C0 2 . When 12 kg. of car- 
bon (that is to say the atomic weight of carbon taken in kilo- 
grams) are oxidized to CO, 29,400 calories are given off, and 
when C0 2 is formed a further 68,200 calories are liberated, 
making a total of 97,600. This means that if the carbon 
be only oxidized to the CO stage not more than about 30 
per cent, of the available heat energy is given up, and that 
by far the most of the available heat is obtained from the 
stage in which CO becomes C0 2 . Even supposing that in 
a given steam boiler the whole of the 97,600 calories were 
given off from each 12 kg. of carbon (neglecting for the 
moment the hydrogen and hydrocarbons in the coal) only a 
fraction, not greater than 60 per cent., ever gets to the 
water, and the balance goes away up the chimney or is lost 
by radiation. With gas producers, however, no such losses 
occur. Their efficiency depends upon the working process, 
but it may be taken as being seldom less than 80 per cent, 
and often as much as 90 per cent, even when working with 
anthracite coal and not chemically pure carbon. In a gas 
producer, air is forced or drawn through a mass of highly 
heated fuel, with the result that the carbon is oxidized. 

173 



174 THE INTERNAL COMBUSTION ENGINE 

Also, in order to keep the temperature within reasonable 
limits, and for another reason to be given later, steam is 
admitted along with the air and both together pass upwards 
through the glowing fuel. 

When the air and steam are forced through by pressure 
the producer is called a Pressure Producer. When however 
they are drawn through by suction caused by the suction 
strokes of the engine, they are known as Suction Producers. 
The theory is in each case the same. 

It may seem strange that the gas given off should 
contain as much as 80 to 90 per cent, of the total 
heat energy in the coal. Those who approach the subject 
for the first time are aware from their knowledge of 
chemistry that even if pure CO came away from the pro- 
ducer, and no C0 2 at all, there would even then be a loss of 
the 30 per cent, of energy given up when the carbon was 
oxidized to CO, so leading to an apparent maximum pos- 
sible efficiency of 70 per cent. The explanation is that the 
30 per cent, is not wasted. It serves to keep the furnace 
alight, and to decompose the entering steam. This steam 
is decomposed into hydrogen and ox} T gen thus 

2H 2 0=2H 2 +0 2 (1) 

and in so doing it stores up 116,400 calories for each 36 kg. 
of water decomposed. It is not difficult to see that 
by balancing the proportions of air and steam admitted it is 
possible to absorb the whole of the 30 per cent, of energy 
rendered available by the formation of CO, and to carry 
it as potential chemical energy to the gas engine where 
the hydrogen and oxygen can again unite. In reality it 
is not quite so simple as this, because the oxygen from the 
decomposed steam has also to pass over glowing carbon, 
with the result that a further supply of CO is formed. 
Radiation of heat occurs also, and this prevents the 
efficiency being 100 per cent. 

Following generally the procedure adopted by Mr. 
Dowson, who invented the first of these plants, the reactions 
may be send-mathematically stated thus : — 

Taking weights equal to molecular weights in kg. 



THE GAS PRODUCER 175 

Carbon-monoxide is thus formed :■ — 

2C+0 2 =2CO +58, 800 Calories .. .. (2) 

Carbon-dioxide would be formed thus : — 

C+O 2 =CO 2 +97,600 Calories . . . . (3) 

The former of these two equations gives a gas having a 
calorific value of about 119 B.T.U. per cubic foot, but when 
steam is admitted this value rises rapidly owing to the 
hydrogen present. 

As already stated the decomposition of steam follows 

2H 2 0=2H 2 +0 2 — 116,400 calories .. (4) 
the negative sign meaning that heat is absorbed and not 
liberated. The oxygen so produced also joins in the re- 
action, so that one of the following formulae 

H 2 0+C=H 2 +CO— 28,800 calories .. (5) 

or 2H 2 0+C=2H 2 +C0 2 — 18,800 calories .. (6) 
is followed in the decomposition of the steam. In both 
(5) and (6) an absorption of heat takes place which allows 
of a balance being obtained by a careful regulation of the 
relative proportions of air and steam admitted. 

83. It is useful to discover what quantity of water is 
theoretically required per pound of coal in order to keep 
this reaction balanced. 

Assume that the reaction follows equations (2) and (6). 

Really it will not follow quite such simple laws, but it will 

approximate thereto if the temperature is high enough. 

Equation (2) shows that for each 24 kg. of carbon used 

58,800 calories will be liberated, and equation (6) that 18,800 

calories will be absorbed by each 36 kg. of steam dissociated, 

requiring also for its dissociation 12 kg. of carbon. To ab- 

58 800 
sorb the whole of the 58,800 calories liberated 36 X— 

18,800 

kg. of steam would be required. But the steam is 
not admitted to the producer as steam, but as water, 
and there is therefore the latent heat of evaporation to 
be considered. Now the latent heat of 36 kg. of water- 
vapour at 20° C. is 21,600 calories, and this must be 
added to the 18,800 calories due to chemical dissocia- 
tion, making a total of 40,400 calories, so that only 



176 THE INTERNAL COMBUSTION ENGINE 

36 x — kg. of water would really be required, and this 

40,400 

works out at 52 4 kg. of water. The quantity of carbon 
corresponding to this is clearly 24+ ( — x524j=24 + 

524 

17-5=41-5 kg. of carbon. So that or 126 kg. of water 

6 41-5 6 

will be required for each kg. of carbon. 

The next point to determine is the nature of the 
mixture of gases given off in this way. Equation (2) 
shows that for each 24 kg. of carbon there will be 
given* off 22-4x2x1,000 litres = 44,800 litres of CO. 
Equation (6) adds to this an equal volume of hydrogen 
and half the volume of C0 2 for each 12 kg. of carbon. 
Now the quantities in equation (6) must clearly be propor- 
tional to 17*5 and not 12 kg. of carbon, and therefore the 

175 

volume of hydrogen will be x 44, 800 = 65, 200 litres and 

i z 

the volume of C0 2 will be 32,600 litres. The total will 

therefore be 

CO :— 44,800 litres 

C0 2 :— 32,600 

H 2 :— 65,200 



142,600 litres or 142-6 cubic 

metres. 

But it must be remembered that in equation (2) oxygen 

is supplied to the extent of 22,400 litres, and that as this 

79 
is drawn from the air it will be accompanied by — x 22, 400 

litres of nitrogen which will pass through without change. 

79 
So that to the above table must be added — x 22,400 = 

2i\- 

84,100 litres of nitrogen, making the total and proportions 
thus : — 

* Based on the principle that the molecular weight of any gas 
taken in grams will occupy a volume of 22-4 litres. (Some recent 
work has been based on a revised figure of 22.25 litres.) 





THE GAS PRODUCER 


CO . 


. 44,800 litres or 198 per cent. 


co 2 . 


. 32,600 ,, or 144 per cent. 


H 2 . 


. 65,200 „ or 288 per cent. 


N 2 . 


. 84,100 ,, or 370 per cent. 



177 



226,700 1000 

Thus 226,700 litres of gas are given off for each 41 5 kg. 
of carbon or, 5,450 litres per kg. of carbon, and 5,450 litres 
is of course 5-45 cubic metres. 

What is the calorific power of this producer gas ? The 
N 2 and C0 2 can do nothing. The CO will yield up 

68 200 

(97,600—29,400) calories for each 28 kg. of CO, or — - 

28 

=2,440 calories per kg. of CO. The H 2 will yield 116,400 
calories per 4 kg. of gas, or 29,100 calories per kg. of hydro- 
gen. Take 1 cubic metre or 1,000 litres of the producer 

198 

gas. It will contain 198 litres of CO yielding x 68,200 

fe J & 22,400 

288 

= 602 calories, and of hydrogen x 58,200 = 750 calories, 

J 8 22,400 

making a total of 1,352 calories per cubic metre. Fur- 
thermore the steam formed by the union of the hydrogen 
and oxygen will be capable of yielding up its latent heat 
which will add 21,600 calories for each 4 kg. of hydrogen 
concerned. Now the weight of the hydrogen in 1,000 litres 

288 

of the gas is x 2 kg. and the calories in the latent heat of 

6 22,400 6 

the steam will therefore be x2 x — =139 calories, 

22,400 4 

which when added to the 1,352 calories found above, makes 
a total calorific value of 1,491 calories per cubic metre of the 
gas given off by the producer. In cases in which the latent 
heat of the steam formed cannot be utilized, it is custom- 
ary to use the lesser value of the calorific constant, and 
write it down in this case as 1,352 calories only, which is 
nearly 10 per cent. less. The figure of 1,491 calories per 
cubic metre corresponds to 168 B.T.U. per cubic foot. 
84. Dowson has carried out calculations similar to the 

N 



178 THE INTERNAL COMBUSTION ENGINE 



above for a number of possible reactions, and the follow 
ing tables show some of the results he has found. 



Reaction 














between 














Air and 














Carbon : 














proportions 
of CO and 

co 2 

formed 

per cent. 

by volume, 

depending 

upon the 

temperature 

of the 


Composition of gas per cent. 

by volume. 

(Steam decomposed according 

to equation (6)). 


Steam 

used 

per 

kilo 

of 

Carbon 


Gas 

formed 

per 

kilo 

of 

Carbon 


Calorific 
Power of 
Gas made. 


reaction. 






























Calories 


B.T.U. 


CO. 


co 2 . 


co 2 . 


CO. 


H 2 . 


N ? . 


Kilos. 


Cubic 
Metres. 


per 
Cubic 
Metre. 


per 

Cubic 
Foot. 





100 


28-45 




40-25 


31-3 


212 


6-54 


1,243 


139-7 


10 


90 


27-8 


0-9 


39-7 


31-6 


2-08 


6-48 


1,254 


140-9 


20 


80 


271 


1-9 


3915 


36-85 


2 02 


6-41 


1,267 


142-4 


30 


70 


26-3 


3 


38-5 


322 


1-97 


6-34 


1,282 


1440 


40 


60 


25-35 


43 


37-7 


32-65 


119 


6-26 


1,298 


145-8 


50 


50 


24-3 


5-85 


36-8 


3305 


1-83 


617 


1,316 


147-9 


60 


40 


230 


7-65 


35-8 


33-55 


1-75 


607 


1,340 


150-5 


70 


30 


21-5 


9-8 


34-55 


3415 


1-66 


5-95 


1,366 


153-5 


80 


20 


19-6 


124 


330 


350 


' 1-55 


5-81 


1,398 


1571 


90 


10 


17-3 


15-65 


311 


35-95 


1-42 


5-65 


1,438 


161-6 


100 





14-4 


19-7 


28-8 


37 1 


1-26 


5-45 


1,490 


167-5 



This table serves to show the very thorough manner in 
which Mr. Dowson has worked out the chemical problems 
relating to producer gas, and the student who wishes to 
pursue such matters further is referred to that writer's very 
able book on the subject. 

We have now discussed the ideal conditions of working. 
In practice about the theoretical weight of water is used in 
suction producers. For pressure producers such as the Mond 
producers an excess of steam is admitted in order that the 
temperature of the coal may be kept to a point lower than 
that at which ammonia dissociates, it being a feature of 
this process to recover and sell the ammonia produced 
from the nitrogen contained in bituminous coals ; the 



THE GAS PRODUCED 



179 



effect of this, incidentally, is to lower the thermal efficiency 
of the producer to about 80 per cent. 

Equation (5) may sometimes be followed instead of equa- 
tion (6) for the decomposition of the steam, depending on 
the temperature of the reaction and the masses involved. 
Mr. Dowson gives these two comparisons of the theory and 
practice in each case : — 



Theory. 
Gas formed according 
Equations (2) and (5) : — 



to 



Per cent. 
by Volumes. 

CO 39-9 

H 2 17-0 

N 2 431 



100-0 



Practice. 
Gas made at Mill wall. 
121-3 vols, contain same weight 
of carbon and consist of : — 

Volumes. 

CO 33-5 

H 2 18-6 

No 62-8 

C0 2 4-7 

Methane . . . . 1-7 

121-3 

Gas made at Winnington. 
117-6 vols, contain same weight 
of carbon and consist of : — 

Volumes. 

CO 12-9 

H 2 34-1 

C0 2 18-8 

N 2 49-4 

Methane .... 2-4 

117-6 



Gas formed according to 
Equations (2) and (6) : — 

Per cent, 
by Volumes. 

CO 19-7 

H 2 28-8 

C0 2 14-4 

N 2 37-1 

100-0 
It will be noticed that an 
excess of air has been admitted 
in each case. 



85. Actual Producers. In Fig. 54 is shown a reproduction 
of a working drawing of a 150 h.p. suction producer made by 
the Campbell Gas Engine Co. The steam required for the re- 
action is derived from the annular boiler surrounding the gas 
producer, and the heat necessary for vaporization is derived 
from the heat of the fuel. This steam passes with the air down 
a pipe leading to the base of the gas producer, and is then 
drawn through the glowing fuel which is maintained at a 
temperature of about 1,000° C. The air and steam on passing 
through the furnace are decomposed in accordance with the 



180 THE INTERNAL COMBUSTION ENGINE 

equations already given, and the hot producer gas then 
passes through a dust trap or separator, and then past a 




»] -»• To Engine . 



Fig 



777777 ;///)// )/ 7777777 A/ //V/vVy j jj ;;/j 



54. — Sectional elevation of a 150 H.P. Campbell Suction Gas Pro- 
ducer. Fuel is first admitted through the hopper B. Air then enters 
at A, picks up steam on its way and passes by the pipe E to the grate 
C The gases come away from the upper part of the producer and 
pass by the pipe system shown at D to the Scrubber Chamber where 
they are cleansed and cooled. The gases are next drawn along the 
pipe F to the expansion box G on their way to the engine. 



water seal into the coke scrubber which consists of a tall 
vertical vessel containing coke upon which a water spray is 
kept playing. This cools the gas, condenses any steam 
there may be in it and serves generally to cleanse it. Thence 
the gas passes to a gas box to equalize the pressure, and from 
that it is drawn into the engine as wanted. A full description 
of how to work such a producer is, on account of its general 
interest to the many readers who will be unacquainted with 
the actual working of such plant, given as an appendix to this 
chapter. The above description applies to a plant using 
anthracite. When it is desired to use coke, a sawdust scrub- 
ber is usually required in addition to the coke scrubber. An 
outside view of a similar plant is also given in Fig. 55. 

There is not a great deal of difference between the different 
makes of suction producer plant. Fig. 56 shows an out- 
side view of a National Gas Engine Co. type, similar to that 



THE GAS PRODUCER 181 

which was awarded the gold medal at the Royal Agricultural 
Society's Trials in 1906. Its internal arrangements are 
very similar to those already described, except that the 
vaporizer is fed with water which has first been heated by 
being passed through a pipe in the gas outflow passage and 
is then vaporized on the " flash " system. 

Pressure producers are worked on much the same general 
principles, except that the air and steam are forced through 
the coal instead of being sucked through. In general, too, 

























'» i 








II 



Fig. 55.— Outside view of 80 B.H.P. Campbell Suction Gas Plant. Note 
small size of Producer for the amount of power produced. 

they are for much larger plants. Suction producers are 
usually for quite small outputs — commonly about 30 or 40 
h.p. and rarely going beyond 500 h.p., whereas the power 
from pressure producers may run into thousands of horse- 
power and the latter are therefore of a much more extensive 
nature, and a good deal more complicated, especially when 
a feature is made of by-product recovery. 

86. Tests. It will be of interest to give here some 
figures from the tests held on suction producer plant in 1 905 



182 THE INTERNAL COMBUSTION ENGINE 

by the Highland and Agricultural Society of Scotland, 
and in 1906 by the Royal Agricultural Society. 

In the 1905 trials ten complete plants, exhibited by six 




m p* 



w. 

C 

oa 

© 

'Sb 
a 

w 
eg 



different firms, were sent in for the competition. Particulars 
of these plants are given in the following table— 



O 
PL, 

CO 

« 

O 

w 

3* 

00 frj 

£° 

o 

M 
<! 

ft 

O 

CO 

H 
fe 

<J 
ih 
Ph 


Messrs. 
Tangyes, 
Ltd., 
Bir- 
mingham. 


CO 


£58 
17 

Tangye 


rH|N 

t— £- CO 


CM <?> tJH 

CO =+* 


£148 
160 


The 

National 

Gas-Engine 

Company, 

Ashton- 

under- 

Lyne. 


o 


£65 

20 

"Nation- 
al" 


GO t^ LO 


° £ ° 

CO °° -h 
co «rt 


£145 
225 


The 
Industrial 
Engineer- 
ing 
Company, 
Hyde, near 
Manchester 


© 


£60 
15 

"Acme" 


CO GO "^ 
r-H i— 1 


© »o io 

CO J> CO 

co «rt 


£135 

288 


The 

Campbell 

Gas -Engine 

Company, 

Halifax. 


00 


£80 
25 

Campbell 


GO t^ CO 


O © Oi 
co oo co 
co «rt 


091 
0913 


H 
Q 
<j 
Pm 
<i 
O 

tf 
H 

o 
Ph 

H 

a 

o 

w 

H 

M 
< 

PQ 
o 

CM 

H 
|3 

o 

«j 

Ph 
O 

CO 
H 

< 

Pw 


Messrs. 
Tangyes, 
Ltd., 
Bir- 
mingham. 


CO 


£90 

28 

Tangye 


ffiOOJ 


© CO OS 
OS r-H CO 
pH CfJ 


£221 
225 


The 

National 

Gas-Engine 

Company, 

Ashton- 

under- 

Lyne. 


o 

CO 


£80 
25 

" Nation- 
al" 


O © GO 
CO r-H r-l 


© 
© CO 00 

pH Crt 


£200 
225 


The Indus- 
trial 
Engineer- 
ing 
Company, 
Hyde, near 
Manchester 


LO 

CO 


£72 
25 

"Acme" 


CO © t- 

C0 PH pH 


© 

© CO © 

co ph co 

CO crt 


£202 
288 


Messrs. 
Crossley 

Bros., 
Openshaw, 
Man- 
chester. 


CO 


£80 
45 

Crossley 


CO GO © 
ph CO 


© 

© PH T* 

Ohio 
CO c+i 


£190 

240 


The 

Campbell 

Gas-Engine 

Company, 

Halifax. 


GO 


£105 
25 

Campbell 


r-i|eq 

GO OJ 00 


>o 

© CO CO 
O ph CO 
CO c+3 


£230 
225 


The Acme 

Engine 

Company, 

Shettleston, 

Glasgow. 


lO 
CO 


£94 10s. 
35 

"Acme" 


CO © t- 

C0 PH pH 


© 

©CO © 
CO ph CO 
CO Crt 


£224 10s. 
144 


1 




Declared capacity of gas- 
producer plant, B.H.P. 

Price of gas-producer 
plant (complete) 

Total weight of plant, cwt. 

General description of 
engine 

Declared brake horse- 


power of engine . 

Diameter of cylinder, in. 

Stroke . . . . ,, 

Revolutions per minute 
(declared) .... 

Price of engine (complete) 

Weight of engine, cwt. 

Price of producer plant 
and engine (complete) 

Space taken up by com- 
plete plant — producer, 
engine, etc. . . sq. ft. 


o 

H 



183 



184 THE INTERNAL COMBUSTION ENGINE 

The result of the trials was given in the Judges' 
report,* of which the following contains an account. 

Each plant was allowed half an hour of steady working 
before the actual power test, at the end of which the plant 
was brought back as nearly as possible to the same condition 
in respect of fuel, etc., as it was at the beginning of the trial, 
and the actual weight of fuel supplied in the interval was 
taken as that consumed by the plant during the power test. 
The obviously weak point in this procedure was that it was 
quite impossible to determine absolutely whether the plant 
was really in the same condition at the end of the trial as it 
was at the beginning. By running the test for a long enough 
time, however, any slight error in this respect could be 
rendered of little importance, and probably the method 
adopted was the best one. The alternative would have 
been to start the producers up from rest, and note the fuel 
put in, then at the end of the trial, note the proportion in 
the producer which had not been burnt, subtract the two, 
and add to this any fuel which had been introduced during 
the test. This procedure was adopted at the R.A.S. trials 
in 1906, except that the fuel consumed when the producers 
were banked up all night was also included, so 
leading to the disadvantage that it did not give a real 
fuel economy test. Also it was extremely difficult to tell at 
the end of the trial how much of the fuel left in the pro- 
ducer could properly be said to be " unburnt." 

In the Scotch trials it was found that the coal used per 
brake h.p. at full load, varied from 1*25 to 084 lb., and at 
half load from 155 to 091 lb. This was for the 8 h.p. sizes. 
For the larger, 20 h.p., plants the fuel used per b.h.p. at full 
load varied from 0- 93 to 77 lb. and at half load from 1 08 to 
92 lb. These results serve to show how economical the 
suction producer plant is when compared with steam engine 
plant of the same output ; the latter would consume any- 
thing from 2J times to 4 times as much fuel per b.h.p. Other 
interesting figures reported by the Judges are that the capa- 
city of the producer per declared b.h.p. varied from 0124 
cu ? ft., to 0295 cu. ft. for the 20 h.p. size, and from 0*161 
* Engineering, November 17, 1905. 



THE GAS PRODUCER 



185 



cu. ft. to 0372 cu. ft. for the 8 h.p. size. Each of these 
figures show a ratio of about 2-3 to 1 and the price of the 
plants varied also but in not so great a ratio. The variation 
in cubic feet capacity per b.h.p. was sufficient indication 
that little had been done towards standardization of 
design. 

87. The tests carried out by the R.A.S. in 1906 were 
considerably more elaborate, and, as already stated, a 
different procedure was followed. The report of the Judges 
had been published and, although in some aspects it may 
be said to be controversial, it is certainly worth study. 
Fourteen plants were entered for trial and all but three 
ran through to the finish. The capacity in each case was 
15 to 20 h.p. A list of the plants with their leading dimen- 
sions and other particulars is given here — 



Name of 


Name of 


Revs. / 


Stroke 


Diam. 


Declared 


Producer. 


Engine. 


min. 


In. 


of Cyl. 
In. 


b.h.p. on 
Anthracite 


National . 


National 


190 


18 


10 


20 


Dowson . 


Railway and 
General 


170 


18 


12 


20 


Paxman . 


Paxman 


220 


15 


9^ 


15-5 


Dowson 


National 


190 


18 


10 


20 


Campbell . 


Campbell 






200 


19 


n 


18 


Campbell . 


Campbell 






190 


20 


10 


20 


Dudbridge . 


Dudbridge 






200 


17 


9| 


20 


Mersey 


Gardner 






200 


18 


9 


20 


Hindley . 


Hindley . 






600 


7 


7 


16 


Kynoch . 


Kynoch . 






240 


18 


9 


17 


Newton . 


Newton . 






200 


18 


9 


20 


Fielding . 


Fielding . 






220 


18 


9} 


18 


Crossley . 


Crossley . 






220 


21 


8} 


17 


Crossley . 


Crossley . 






180 


21 


H 


15 



Measurements made of the fuel and water consumption 
showed figures ranging from 147 to 104 lb. of anthracite 
per b.h.p. -hour and from 3 61 to 73 gallons of water per 
b.h.p. -hour. The enormous variation in the quantity of 
water required was very striking, and it showed that there 
was a considerable difference in the manner of operation of 
the various plants. As the water required for steam making 
is very small, practically the whole of the above difference 



186 THE INTERNAL COMBUSTION ENGINE 

must have been due to the different quantities taken by the 
scrubber. 

The Judges published the following conclusions as a result 
of the consumption trials — ■ 

That with a good suction producer plant, working contin- 
uously, at the specified loads and under the best conditions 
the following results may be anticipated : — 

With Anthracite. 

Full load : 11 lb. per b.h.p.-hour including fuel needed for 
starting, and for banking during the night. 

Half load : 1*6 lb. per b.h.p.-hour including as above. 

Water : 1 gallon per b.h.p.-hour at fuU load and f gallon 
at half load. 

With Coke. 

Full load : 13 lb. per b.h.p.-hour including fuel needed for 

starting. 
Water : 1 J gallons per b.h.p.-hour at full load. 

Professor Dalby * also recorded as a result of these trials 
that — ■ 

" Assuming a 20 b.h.p. plant to start on Monday morning 
with an empty producer, and to run ten hours per day on 
full load for a week, banking the fires at night, the consump- 
tion of anthracite peas would be about half a ton for the 
week, and about f ton if the average load is about half full 
load. With coke the consumption is about 25 per cent, 
more. From 2,000 to 3,000 gallons of water per week are 
required for a 20 b.h.p. plant to provide water for the 
scrubber and the producer, and of this by far the larger part 
would be used in the scrubber." 

Tests were also made of the times taken to start up and 
to change load. As a result of their investigations the 
Judges awarded the premier places to the National and 
Crossley plants. The Judges were Professor Dalby and Capt. 
Sankey, R.E. 

88. Test of a Dowson Suction Gas Producer Plant.— The 
following account of tests on two Dowson Suction Plants is 

* B. A. paper, August, 1906. 



THE GAS PRODUCER 187 

extracted from Mr. Dugald Clerk's 1904 "James Forest" 
Lecture before the Institution of Civil Engineers. The tests 
were carried out by Mr. M. Atkinson Adam, B.Sc, Assoc. M. 
Inst. C.E. The first plant was adapted for a working load 
of 40 b.h.p. and the second for 30 b.h.p. In each case the 
producer was started up cold, and run on test for fully eight 
hours. At the start air was blown in by a small hand-power 
fan and after ten minutes from lighting up the gas was of a 
proper quality. The gas was then sucked through by a 
fan which represented the action of a gas engine operating 
under a constant load sucking gas from a producer in the 
usual way. Thence the gas passed to a gas holder. Analy- 
sis samples were frequently taken and the anthracite analy- 
ses were undertaken by Mr. Bertram Blount F.I.C., Assoc. 
Inst. C.E., whilst the gas analyses were carried out by 
Mr. Horatio Ballantyne, F.I.C. The heat efficiency of the 
producers was found in two ways : — 

(1) Counting in the fuel used in the starting up operation 

which includes that necessary for the heating up of 
the plant. 

(2) Omitting the first two hours of the test, and so giving 

the plant what may be termed a " flying start." 
The quantities of water used are very interesting. The 
figures showed that for vaporization, the 40 b.h.p. plant 
used about 30 lb. per hour, whilst the 30 b.h.p. plant used 
about 20 lb. per hour. For the scrubber, the 40 b.h.p. 
plant used about 400 lb. per hour, and the 30 b.h.p. plant 
used about 380 lb. per hour. This shows how small a pro- 
portion of the total water consumption is needed for vaporiza- 
tion. The anthracite used was of an ordinary commercial 
kind, costing 14s. Qd. per ton at the pit, and about 24s. per 
ton delivered at Basingstoke. The efficiency figures for the 
two producer plants were found to be 

a Qi j- j. j. jj ("40 b.h.p. 85 per cent. 

Standing start . . / Mr * 

( 30 b.h.p. 75 per cent. 

"Flying start" . . f f? "**" 89 P ei ' cent - 
J 8 (30 b.h.p. 86 per cent, , 

Reference should be made to the paper for detailed figures. 

but it may be mentioned that the gas was found on a general 



188 THE INTERNAL COMBUSTION ENGINE 



average to have a calorific value of 135 B.T.U. per cubic 
foot, and have a composition as follows : — 




oo pei 


12 „ 


200 „ 


70 ,-, 


Oo „ 


55-8 „ 



cent. 



1000 
89. Tests of Pressure Producers. — In 1 904 some exhaustive 
tests were made in America on the results of employing 
different varieties of bituminous coal in pressure producer 
plants and in steam engines, and it is worth while to give a 
brief account * of some of the figures obtained. 

















Coal burned 
per E.H.P. 


Ratio 
of Coal 
used by- 






Steam 


Kind of Coal. Xarne of Samjale. 






Plant 

fn that 




Steam 
Plant. 


Gas 

Plant. 


LU I HO. u 

used in 
Gas 






Plant. 




Lb. 


Lb. 




Bitumin . | Alabama, No. 2 . . . 


4-08 


1-64 


2-49 


Black lignite 




Colorado, No. 1 . 






4-84 


1-71 


2-83 


Bitumin . 




Illinois, No. 3. 






4-34 


1-79 


2-42 


)> 








,, „ 4. . 






4-80 


1-76 


2-73 










Indiana, No. 1 




413 


1 93 


214 


53 








» 2 . 






4-35 


1-55 


2-81 


>5 








Ind. Terr., No. 1 . 






4 04 


1-83 


2-21 


«» 








„ 4. 






4-64 


1-43 


3-24 


J) 








Iowa, No. 2 






4-95 


1-73 


2-86 










Kansas, No. 5 






3-93 


1-62 


243 


J5 








Kentucky, No. 3 . 






4-22 


1-91 


2-21 


?5 








Missouri, No. 2 






4-93 


1 71 


2-88 










W. Virginia, No. 1 






3-90 


1-57 


2-48 


) ) 








„ 4 






3-62 


1-29 


2-80 










» 7 






3-55 


1-46 


2-43 


5> 








„ 8 






3-63 


1-78 


2 04 










„ 9 






3-46 


1-40 


2-47 


»5 








» 12 






3-53 


1-50 


2-35 


5> 








Wyoming, No. 2 . 






5-90 
Avera 


2-07 


2-85 
















2-57 



* The Times Engineering Supplement, January 23, 1907. 



THE GAS PRODUCER 189 

In each case the output was about 200 e.h.p., and in 

most cases the length of the trials was from 10 to 30 hours. 

Mr. Shober Burrows has reported the result of a 24 day test 

undertaken in 1 906 on a pressure producer plant operating 

with bituminous fuel. Analysis of the fuel showed — 

H 2 14-68 

Volatile combustible 3098 

Fixed carbon 4293 

Ash ^ 1008 

S 133 

10000 

B.T.U. per lb =12,343. 

The gas left the generator at about 644° F. and passed a 
water seal to the scrubber. Thence to a centrifugal tar ex- 
tractor. The calorific value of the gas was found to be 156 
B.T.U. per cu. ft. and its composition was 

C0 2 9-2 

Ethylene 04 

CO . 209 

H 2 15 6 

Methane 19 

N 2 520 

1000 
About 143 lb. of tar was extracted per ton of coal used in 
the producer, whilst the approximate figures show that an 
average of 139 lb. of coal was used per b.h.p.-hour. As 
this plant ran for 24 consecutive days without shutting 
down, it is evident that continuity of operation can be 
practically achieved. 

The whole of these tests go to show the great fuel economy 
obtained by the use of gas plant as contrasted with steam 
plant. Another feature in which the gas plant has the 
advantage is in the smallness of the stand-by losses. When 
a boiler is banked up for the night it consumes a very much 
larger quantity of coal during the period of banking than a 
producer plant of the same output would require. Actual 
measurements of this nature are recorded by Mr. Dowson 



190 THE INTERNAL COMBUSTION ENGINE 

in his book on Producer Gas, and as a result it was found 
that in the case of steam power, the consumption of fuel per 
standing hour was 71-5 lb., and in the case of gas power, 
3-5 lb. only, which shows a ratio of about 20 to 1. And since 
each of these figures is the mean of several tests, they are 
not open to the criticism that they represent isolated cases 
only. 

It will be of advantage to record at this point what are 
the chief objects to be achieved in the design and working 
of producer plant — 

(a) A fairly deep fuel bed should be allowed for, otherwise 

the air may blow through in thin places, and so lead 
to local variations in the temperature. 

(b) Provision of some sort must be made to prevent caking 

or cavitation of the fuel. 

(c) Fuel must be fed in and ashes removed in such a way 

as not to render the process discontinuous or 
intermittent. 

(d) Leakage of gas from pressure producers must at all 

costs be avoided, as the gas contains a large pro- 
portion of poisonous CO. 
There are a good many makes of pressure producer plant, 
and some are adaptable for by-product recovery. Among 
the latter one of the most prominent types is the Mond pro- 
ducer, which is being used on so large a scale in South Staf- 
fordshire. Here ammonia in the form of ammonium sulphate 
(Am. 2 S0 4 ) can be produced as a by-product and sold for a 
considerable amount — often more than enough to pay the 
coal bill. In this process, as has already been explained, 
the temperature of the producer must be kept low, and to 
do this, large quantities of steam are used, as much as 2 J lb. 
of water per lb. of coal. This has the effect of course of 
reducing somewhat the actual efficiency of the gas producer 
and of raising the percentage of hydrogen present, but not 
to such a point as to introduce trouble in the engine. 

90. Percentage of Hydrogen. — The percentage of hydro- 
gen present in the gas to be employed in a gas engine 
regulates the amount of compression which can be used. 
A good compression is essential for high efficiency, but 



THE GAS PRODUCER 



191 



if the proportion of hydrogen is over 30 per cent, the 
danger of pre-ignition has to be looked out for. The 
following table taken from a paper by Mr. J. R. Bibbins* 
shows the proportion of hydrogen present in various 
kinds of gas and the calorific value of the gas when taken 
alone, and when taken with its theoretically requisite 
proportion of air — 



Gas. 



Natural Pittsburg . 

Oil 

Coal-gas .... 
Carburetted water gas 
Water gas 
Producer, hard coal 

,, soft . 

,, coke . 



B.T.U. per cu. ft. 


Gas. 


Mixture, f 


978 


910 


846 


930 


646 


91-7 


575 


920 


295 


88-0 


144 


68-0 


144 


65-5 


125 


630 



H 2 — per 

cent, by 
volume. 



3 

320 
46 
40 
48-0 
200 
100 
100 



Attempts have been made to reduce the proportion of 
hydrogen by admitting some of the exhaust gases into the 
producer instead of water vapour. In this case the dissocia- 
tion of CO 2 replaces that of H 2 0. This process is called the 
" straight carbon-monoxide gas producer." It is claimed 
to work very well and to permit of very high compressions 
being used. The gas has a calorific value of 105 B.T.U. and 
a composition of : — 

26 95 per cent. 

0-20 

1-75 

0-50 
69-30 

1-30 
—The following interesting com- 



CO 
H 2 

co 2 

CH 4 

N 2 
2 

91. Comparison of Costs. 



* "Fuel Gas for Internal Combustion Engines," Gassier s Maga- 
zine, 1906. 

f Based on theoretical air for combustion. 



192 THE INTERNAL COMBUSTION ENGINE 



parison has been drawn up by Mr. L. Andrews * and is well 
worth study. 

Capital Cost of 16,000 K.W. Plant. 



Steam 
Turbines. 



Gas 
Engines. 



Engines and electric generators 

Boilers, feed-pumps, coal, handling 
plant, etc 

Producers, gas-cleaning and coal hand- 
ling plant, with all pipes. 

Engine-room, building, cranes, and en- 
gine foundations 

Switch-gear and wiring for ditto . 



£ 
96,000 

81,000 



18,000 
5,250 



£200,250 
Allowance for contingencies, 5 per cent. 10,012 



Capital cost per K.W. installed 



£210,262 
£131 



£ 
161,700 



77,700 

42,000 
5,250 



£286,650 
14,332 



£300,982 
£18-88 



Running Cost on 100 per cent. Load Factor. Annual 
Output =140.000,000 K.W. Hours. 



Fuel, 165,000 tons at 10s 

Fuel, acid, stores and repairs for pro- 
ducers, less sale of by-products . 

Labour 

Repairs of turbine plant, including 
boilers, etc 

Repairs of gas plant (excluding pro- 
ducers) 

Oil, waste, and stores (excluding pro- 
ducer stores) 

Interest and depreciation at 10 per cent. 

Total cost per K.W. hour .... 



Steam 
Turbines. 



£ 

82,500 



7,000 

8,750 



1,750 
21,026 



£121,026 
0-204d. 



Gas 

Engines. 



28,250 
9,000 



6,000 

4,370 

30,098 



£78,118 
0135d. 



* Electrical Engineering, October 24, 1907, and S.A., January 30, 
1908. 



THE GAS PRODUCER 193 

Mr. Andrews also takes the case when the load factor is 
only 15 per cent., and in that condition of running the costs 
per K.W.-hour came out at 0545^. for steam turbines, and 
0566^. for gas engines. These rates are nearly the same, but 
with rise of load factor the balance would soon turn in favour 
of the gas plant. The Author considers Mr. Andrew's esti- 
mate of the capital cost of the gas plant, viz. nearly £19 per 
K.W., unduly high. 

92. The use of Gas Plant for Marine Propulsion has been 
recently discussed before several engineering societies. 

Mr. J. T. Milton in his 1906 paper before the Institution 
of Civil Engineers stated that he was led to give attention 
to engines of this kind in connexion with proposals to fit 
them in vessels classed with Lloyd's Register. The paper 
deals with engine problems only, and assumes that a proper 
and suitable type of producer capable of using cheap fuel 
will before long be available. The writer of the paper 
specifies the following conditions which must be satisfied 
by a successful marine engine — 

(a) The engine must be reversible. 

(b) It must be capable of being stopped quickly and of 

being started quickly either ahead or astern. 

(c) It must be capable of being promptly speeded to any 

desired number of revolutions between dead slow 
and full speed, and of being kept steadily at the re- 
quired speed for any length of time. " Dead slow " 
ought to be not faster than one-quarter of full speed, 
and should be less than this in very fast vessels. 

(d) It must be capable of working well, not only in smooth 

water, but also in heavy weather, in a seaway in 
which the varying immersion of the propeller causes 
rapidly changing conditions of resistance. 

(e) All working parts must be readily accessible for over- 

hauling, and all wearing surfaces must be capable 
of being promptly and readily adjusted. 
(/) The engine mUst be economical in fuel, and especially 
so at its ordinary working speed. 

Certainly no existing engine complies with all these con- 

o 



194 THE INTERNAL COMBUSTION ENGINE 

ditions, and reference should be made to Mr. Milton's paper 
for a discussion of the difficulties : some curves are there 
given showing the different turning moment curves for 
different arrangements of engine. 

Another paper is that read by Mr. J. McKechnie before 
the Institution of Naval Architects in 1907. Its title is 
" Propelling and Ordnance Machinery of Warships," and a 
portion of it deals with gas engine propulsion. It is stated 
that at the Vickers Works at Barrow-in-Furness there have 
been constructed internal combustion engines of a power 
equivalent to about 40,000 i.h.p., and that for three or four 
years almost continuous research work has been undertaken. 
As a result of the experiments a 2-stroke engine has been 
adopted. This engine, it is claimed, can be worked by pro- 
ducer gas, heavy oil, or compressed air, is reversible, and can 
take gas direct from a pressure producer without any scrub- 
bing being necessary. To prevent the poisoning of the crew 
by the leakage of the gas from defective joints the pipes are 
jacketed with air under compression. 

Not only would the introduction of gas engines for war- 
ship propulsion lead to a gain of space and dead weight 
(so allowing the offensive or defensive materiel to be 
added to), but the better disposition of its parts, and the 
absence of funnels would admit of a great improvement in 
respect of an actual increase in the number of guns which 
could fire on either broadside. In the proposed plan of 
battleship construction, the gas producers are shown divided 
into two sets well on either side of the ship, and the pro- 
pelling machinery is shown well aft. The deck is clear for 
gun barbettes. Mr. Milton gives the following comparative 
table illustrating the superiority of the gas engine plant 
so far as area occupied, weight and fuel consumption are 
concerned — ■ 



THE GAS PRODUCER 



195 



Comparison of Weights, etc., of Steam, Gas and Oil 
Machinery for 16,000 h.p. Battleship. 





Steam Engine. 


Gas Engine. 


Oil Engine. 


I.H.P. available for pro- 








pelling the ship . 


16,000 


16,000 


16,000 


Weight of machinery, 








including usual auxil- 








iaries, but not deck 








machinery. 


1,585 tons* 


1,105 tons t 


750 tons{ 


I.H.P. per ton of ma- 








chinery .... 


101 


1448 


21-33 


Area occupied by ma- 








chinery : Engines and 








boilers or producers . 


7,250 sq. ft. 


5,850 sq. ft. 


4,100 sq. ft. 


Area per I.H.P. . 


0453 sq. ft. 


0-366 sq. ft. 


0-257 sq. ft. 


Fuel consumption in lb. 








per I.H.P. hour — 








At full power . 


1-6 lb. 


101b. 


0-6 lb. 


At about £ full power 


1-66 lb. 


115 lb. 


0-75 lb. 



A further paper is Mr. A. Vennell Coster's before the 
Manchester Association of Engineers, dated 1907. Mr. 
Coster was fifteen years with the marine steam engine, three 
years at sea with the P. & 0. and eleven years with Messrs. 
Crossley Bros., so that his experience gives his conclusions 
no little authority. The following are the advantages 
claimed for the gas engine — 

1. The ship driven with half the amount of fuel. 

2. Standard losses reduced over 75 per cent. 

3. Working pressure confined to the engine cylinders. 

4. No boiler tubes or main steam pipes to burst, nor 

furnace crowns to collapse. 

5. No priming in a heavy seaway, or water hammer in 

pipes and cylinders. 

6. No more difficulties with the firing of boilers on a beam 

sea. Gas producers may be charged only twice every 
twenty-four hours and the rolling and pitching of 
the vessel is rather an advantage than otherwise in 
assisting the fuel down from the charging hoppers. 

* Includes water in boilers. 

,, ,, jackets and piping, but net coal in producers, 

X » j> jackets and piping. 



■ 



196 THE INTERNAL COMBUSTION ENGINE 

The three main difficulties in the way are — 

(1) The construction of a gas producer able to gasify all 

grades of bituminous coal. 

(2) A simple method to cleanse the gas from tar, either 

before the introduction of the fuel into the producer 
proper ; when in the producer ; or after the gas 
has left the producer on its way to the engine. 

(3) Perfect control of the gas-propelled vessel in starting, 

stopping, reversing and running at all speeds. 

The first of these difficulties obviously is avoided if coke 
or anthracite is used in the producer, but this solution is 
neither economical nor satisfactory on other grounds. Bitu- 
minous coal must be regarded as the source of the power to 
be used for ship propulsion. Mr. Coster stated that in his 
scheme for the cargo vessel Lord Antrim the producers 
were worked by means of a down-draught at the top, and an 
up-draught at the bottom, which metat the centre and the gas 
was drawn off by suction. The gas was then thoroughly 
sprayed and cleaned by being passed through coke, sawdust 
and wool wood scrubbers. 

The reversing difficulty can be met in small engines by 
the use of a reversible propeller, but for obvious reasons this 
would not do in the case of large engines. For powers up to 
500 h.p. gearing may be introduced to effect a reversal in 
the direction of propeller relation, just as in a motor car, 
but this cannot be used when the power transmitted is really 
large. 

One of the great difficulties in connexion with the utiliza- 
tion of the gas engine on board ship lies in the fact that 
when the speed of the ship is decreased, the resistance to 
motion is decreased at a far greater rate, and this means 
that the mean effective pressure on the piston must be 
capable of very considerable reduction. When an attempt 
is made to get very low mean effective pressures in a gas 
engine, the engine is liable to stop altogether — in fact the 
gas engine as at present devised is not sufficiently elastic 
in its manner of working to make it an effective rival to the 
steam engine for marine purposes. The difficulty may be 
solved by driving generators from the gas engines, so pro- 



THE GAS PRODUCER 



197 



ducing electric current which can be used in motors driving 
the screw propellers, but this requires a great weight of 
machinery, and is costly. 

93. The well-known firm of Thornycrof t have been working 
a good deal at the problem of adapting gas engines to ship 




Fig. 57. — Two-cylinder Suction Gas Engine and Producer for Marine 
Purposes. (By courtesy of Messrs. J. I. Thornycrof t & Co.) 



propulsion, and illustrations are shown in Figs. 57 and 58 of 
the engines they are interested in. The chief difficulty is to 
devise a suction producer which will work with bituminous 
or caking coal without the necessity of being provided 
with apparatus for the extraction of tar and other by-pro- 



198 THE INTERNAL COMBUSTION ENGINE 

ducts. The tar often amounts to 4 or 5 per cent, and may 
be as high as 15 per cent. It is therefore necessary to 
arrange the producer so that all the tar produced is con- 
sumed before it leaves the producer. This can be done by 
feeding in the fresh fuel from below, so that the heavy hydro- 
carbons given off from it are consumed as they rise into the 
hotter part of the fire. To save weight and space Herr 
Capitaine has hit on the idea of cleaning the gas by intro- 
ducing a fine water spray, which mixes with the dust and 
other impurities, making a kind of fog. This fog then passes 
into a centrifugal machine which is driven fast enough to 
throw out the impurities and leave clean gas in the middle, 
which is then drawn off by the engine. Mr. J. E. Thorny- 
croft * has given the composition of such gas as follows : — 

C0 2 6 per cent. 

CO 25 per cent. 

CH 2 1 per cent. 

H 2 14 per cent. 

N 2 54 per cent. 

Too 

He also remarks that " it will be realized that the size of 
the producer for a given power is comparatively small when 
it is known that the area of the fire grate necessary is only 
005 sq. ft. per h.p., whereas the average for an ordinary 
natural-draught steam boiler, burning 15 lb. coal per sq. ft. 
grate area, would be 02 sq. ft. per h.p." 

The following test result is recorded by Mr. Thorny- 
croft : — " Tests were made on November 8, 1904, with 
the Gashtg No. 1 and JElfriede, & steam tug of very 
nearly the same dimensions and power. The Gastug No. 1 
is 44 ft. 3 in. long by 10 ft. 6 in. beam, and is fitted with 
one of the four-cylinder 70 h.p. suction gas plants. The 
Elfriede is 47 ft. long by 12 ft. beam, and is fitted with a triple- 
expansion steam engine developing 75 h.p. At the towing 
meter the Gastug No. 1 attained a maximum pull of 2,140 
lb., and the Elfriede a maximum of 2,020 lb. A run from 

* Paper on " Gas Engines for Ship Propulsion," read April 5, 
1906. 



THE GAS PRODUCER 



199 




200 THE INTERNAL COMBUSTION ENGINE 

Hamburg to Kiel and back was made by these two boats, 
during very stormy weather, at a maintained speed of 8 J 
knots. The consumption of fuel was measured for a period 
of 10 hours, and was as follows — For the Gastug No. 1, 530 
lb. of German anthracite : for the Elfriede, 1,820 lb. of 
steam coal. This shows an economy of 1 to 3-44 in favour 
of the gas plant." 

EXAMPLES. 

1. Suppose that for 1-2 lb. of coal we get 1 brake horse- 
power-hour from a gas engine using Dowson gas. This 
works a reversed heat engine taking in heat h, in air at 10° C, 
and giving out heat H at 20° C, the mechanical efficiency of 
the engine being 85 per cent. Find H per lb. of coal and 
compare it with direct heating. The calorific power of the 
coal being 8,200 Centigrade heat units. (B. of E., 1899.) 

2. Describe any non-luminous gas- making plant for use 
with a gas engine, working to, say, 100 indicated horse-power. 
What chemical actions take place in the gas manufacture ? 
What is the composition of the gas ? (B. of E., 1899.) 

APPENDIX A 

The following is a description of the operation of a typical 
suction producer plant. 

The suction type of gas-producing plant in question (Campbell) 
consists essentially of two main elements, a gas producer and a 
gas scrubber. In addition to these there is a simple form of 
separator through which the gas passes on its way from the 
producer to the scrubber, and in which it deposits the heavier 
particles of dust which are carried over from the producer. A 
gas box is also provided between the scrubber and the engine, 
to act as a reservoir, from which the engine can draw a regular 
supply of gas. 

1. Method of Gas Production.— In this method of gas production 
air and steam at atmospheric pressure are drawn through in- 
candescent fuel by the motion of the engine, the oxygen, hydro- 
gen and carbon combining in the producer to form a combustible 
gas which is suitable for power purposes. No boiler for providing 
steam under pressure is required and no gasometer, the engine 
generating its supply of gas by the motion of the piston in the 
cylinder. The fuel used must be anthracite coal or coke 



THE GAS PRODUCER 201 

(bituminous coal must not be used). The steam is generated in 
an evaporator which is heated by the fire in the gas producer. 
The air is drawn into the producer over the surface of the 
heated water in the evaporator and in passing takes up the 
steam which it then carries through the producer. 

2. General Instructions. — The coal used should be passed through 
a sieve and no pieces under J in. (5 mm.) should be used. The 
most suitable size is f in. to 1 in. (16 mm. to 25 mm.). Coal dust 
is not on]y of no value, but it tends to stop up the pipes and 
interfere with the working of the plant. The fuel should not 
be moistened before it is used. All the moisture required should 
be provided in the form of steam and pass through the fire 
in the ordinary way as described below. The evaporator 
should always be kept full of water to the overflow pipe. 
A water supply must be provided for the evaporator and the 
coke scrubber, and a drain to carry away the water from the 
scrubber. In any installation of this type, the engine should be 
erected as close to the producer as practicable so that the 
connecting pipes between the two are as short and direct as can 
be arranged. 

3. To start the Gas Producer after Erection or Cleaning. — Before 
starting it is of the greatest importance to see that all the 
piping, cocks, and various vessels which go to make up the gas- 
producing plant should be air tight, as the apparatus when in 
operation is subjected to an excess of atmospheric pressure 
from without. If the various parts of the plant are not air 
tight, the air which leaks in will interfere with the quality of 
the gas and make it poorer. For this reason the whole apparatus 
should be tested after erection to prove the soundness of the 
joints, the test being carried out as follows : Referring to the 
illustration, if all the openings are closed except the cock 
B, and air is then blown into the apparatus by the hand 
fan A the various joints can be tested with a light. If air or 
gas escapes from the joints it will be at once detected. This 
test should be made periodically to see that everything is in 
order. It may be carried out at any time after cleaning, and 
when everything is proved to be in good working order the engine 
should be made ready for immediate use when the gas is available. 

Provided that all the joints are sound and tight, the water 
should now be turned on to the coke scrubber by means of the 
tap C until it overflows through the pipe I) provided for that 
purpcse. It is essential that the scrubber should contain sufficient 
water to seal the gas inlet. 

Water should then be admitted to the evaporator F by means 
of the tap G until it just overflows in drops by the pipe T pro- 
vided for that purpose. This overflow should be very slight 
before starting and must be regulated from time to time when 



202 THE INTERNAL COMBUSTION ENGINE 

running according to the load on the engine as described below. 
The taps C and G and the cock H should now be closed. The cock 




B and the cock J on the waste pipe K should be opened. The 
fire door L and the ashpit door M should then be opened and 
a fire of wood or coke started in the gas producer. Ordinary 



THE GAS PRODUCER 203 

bituminous coal must on no account be used. When the fire is 
burning up well anthracite coal should be added through the 
hopper N in small quantities from time to time as the whole 
mass of fuel becomes incandescent throughout, this being con- 
tinued until the gas producer is full to the level of the bottom 
of the gas pipe P. The fire door L and ashpit door M should be 
closed as soon as the coal is well alight. The hand fan A must be 
used for the purpose of blowing up the fire when starting, the whole 
of the products of combustion being blown by its means through 
the gas pipe P, separator R and uptake pipe K to waste, the 
cock J being open during this operation. Supposing the fire to 
have been lit for, say, fifteen to twenty minutes, and the hand 
fan to have been in operation during that time, the quality of 
the gas which is being made can now be tested by partially 
closing the cock J and thus passing the gas through the scrubber 
and gas box to the test cock Q. The nearer this test cock 
is placed to the engine the better ; it can be placed at any 
convenient point in the gas pipe, between the engine and the 
gas box, for example. Before passing the gas through the scrub- 
ber the water must be turned on to the scrubber, by the tap C. 
The blowing will have to continue for a few minutes until the 
scrubber and gas box are cleared of air, and gas has been blown 
in to take its place. A fight should then be placed to the test 
cock Q and the gas if of a good quality will burn with a steady 
flame. If the coal is of good quality the gas will burn with a 
long flame, orange red in colour, and one which does not go out. 
With some coals it is difficult to produce anything but a blue 
flame, but as long as the gas burns steadily it will generally be 
found that it is of sufficiently good quality to start the engine. 

Caution. — When testing the gas, as described, care must be 
taken to turn the fan at a steady and even speed. Under no 
circumstances should the fan be stopped while the gas is burning 
at the test cock or the pressure will at once fall and the flame 
will probably be drawn back into the gas box and fire the gas 
in the gas box and scrubber, the explosion caused thereby 
blowing the water out of the water seal and possibly doing other 
damage. On the other hand the fan must not be blown too hard 
or the gas will be forced out through the water seal at the bottom 
of the coke scrubber. 

The tap C should be opened to such an extent that the tem- 
perature of the lower portion of the scrubber does not rise above 
100° Fahr. approx., the top of the scrubber being cold. The 
amount by which the tap G is opened must be regulated accord- 
ing to the load on the engine and so that the evaporator is 
always full and a slight surplus of water runs in drops only 
through the overflow pipe T into the ashpit when the engine is 
running under a full load. When running under a light load 



204 THE INTERNAL COMBUSTION ENGINE 

little or no water is required in the ashpit. An excess of water 
in the ashpit results in a poor quality of gas. 

4. To Start the Engine. — As soon as the gas is burning satis- 
factorily at the test cock this cock and the cock J should be 
closed and the fan stopped. The engine should then be started 
in the usual way. No time must -be lost in getting the engine 
started or the fire in the producer will become dull and a poor 
quality of gas be given off. Assuming that the engine has been 
started, the cock H should be opened and more water turned on 
to the scrubber by the tap C and to the evaporator by the tap 
G. The cock B should now be closed. By opening the cock H 
and closing B the air is drawn in through the inlet S and over 
the heated water in the evaporator F, the suction set up by the 
movement of the engine piston causing a constant indraught of 
air in the direction shown by the arrows in the sectional diagram. 
It may be mentioned here that the supply of air to the engine 
will have to be adjusted from time to time according to the 
quality of the gas. For this purpose a simple form of throttle 
valve should be provided in the air inlet passage through which air 
is supplied to the engine. This valve should be regulated so 
that as far as possible the engine takes in a supply of gas at 
every cycle and thus keeps the fire in the producer bright and in 
good condition. The engine should be provided with mechanism 
to ensure this being done. 

5. Method of Stoking the Gas Producer. — The gas producer is 
provided with a hopper at the top for the purpose of feeding the 
fire. The hopper is provided with a swing door at the top and 
a valve with a weighted lever at the bottom so that when fresh 
coal is added the top door only is opened, the valve remaining 
closed. When the coal has been filled in through the hopper 
the top door is closed and the valve opened. By this means 
all air is excluded from the gas producer. Care should be taken 
to see that the valve to which the weighted lever is attached is 
properly closed so that no air can enter while the gas producer is 
working. Generally speaking it will be necessary to add a 
charge of anthracite every two or three hours ; this, how- 
ever, must depend upon the size of the apparatus and the 
amount of power which the engine is developing. While 
the gas producer is in full operation the coal should not be 
allowed to fall below the lowest point in the evaporator. The 
top layer of coal should never be incandescent, this point can 
be watched through the mica window which is provided for 
that purpose at the top of the hopper. Previous to stopping 
the engine, however, the fire in the producer should be burnt 
down so as to leave only a moderate quantity of coal in the 
producer, sufficient to start up quickly again when required. 
How frequently the fire will have to be cleaned will depend 



THE GAS PRODUCER 205 

upon the quality and amount of the fuel used ; speaking generally 
twice a day will be sufficient, once in the morning before starting 
and once at midday, if a stoppage is made then. Should it be 
necessary to stir up the fire whilst the engine is at work this can 
be done through a hole in the ash door by means of a poker. 
If it is necessary to take out the clinker whilst the engine is 
at work, this should be done very quickly so as to allow as little 
air as possible to enter the gas producer, for should an excess 
of air be allowed to enter, the gas would be of inferior quality. 
It is advisable as far as possible to leave the gas producer alone 
whilst the engine is at work, except for the occasional charges of 
coal which it requires. The gas producer should be cleaned out 
entirely about once a week and the clinker chipped off the 
firebrick lining of the producer if necessary. The producer 
should never be cleaned directly after the fire is raked out, 
but should be allowed to cool down gradually, otherwise the 
firebrick lining will probably crack through the rapid change in 
temperature. 

6. Hydraulic Box. — The surplus from the coke scrubber is led 
into the hydraulic box W by means of the pipe D. This water 
forms at the same time a water seal for the pipe which connects 
with the separator mentioned above. The box W should be 
cleaned out every few weeks so as to keep it clear of the 
accumulated ash and small particles of coal which will 
come over with the gas. Special attention should be paid 
to the pipe D to see that no foreign matter settles in 
it. When the engine is at work the surface of the water in 
the hydraulic box will be in constant movement ; the move- 
ment which should be a slight one, will vary with the amount of 
gas drawn away by the engine. An overflow pipe X is pro- 
vided to run the water away from this box to a drain, or as 
may be arranged. 

7. Coke Scrubber. — The coke scrubber is provided to remove 
from the gas all its impurities and at the same time to cool it. 
When the apparatus has been erected the inside of the scrubber 
should be thoroughly cleaned and the grating put in through 
the upper manhole. The scrubber should then be filled with 
well washed foundry coke, the size being not less than about 
1 in. (25 mm.). The bottom layer of coke for a depth of about 
8 in. (0 - 2 m.) should consist of pieces which are under any cir- 
cumstances so large that they will not fall through the grating. 
The scrubber can then be filled up with coke to about 4 in. 
(0-1 m.) below the water pipe. Before starting the bottom of the 
scrubber should be cleaned out through the doors provided for 
that purpose. All the openings in the scrubber should now be 
closed and the water supply turned on so that the coke is washed 
thoroughly free from all the particles of dust which it may contain. 



206 THE INTERNAL COMBUSTION ENGINE 

Every three or four weeks the bottom door of the coke scrubber 
should be opened to see whether there is any accumulation of dust 
in the form of mud at the bottom of the scrubber ; this if present 
should be removed. When the coke is first put into place this 
examination should be made more frequently, as new coke 
frequently contains a large quantity of dust. The coke in 
the scrubber will, generally speaking, be serviceable for a 
period of nine to twelve months, but this depends upon the 
amount of work which the plant has to do. When it is found 
necessary to renew the coke in the scrubber the whole of the 
apparatus must be stopped, the waste pipe opened and all the 
ash and fire hole doors opened and left open for several hours 
before any work is done to the plant. The upper cover of the 
scrubber should then be removed and the coke taken out through 
the upper side door in the scrubber. This cleaning should take 
place during the daytime so that no fire or light need be brought 
into the gas-plant house while it is going on. The windows of 
the house should be open during the process of cleaning so that 
there is plenty of ventilation. It is advisable that there should 
always be two men present during the operation of cleaning, 
in case one of them should be overcome by the presence of gas. 
When replacing the doors on the scrubber after having renewed 
the coke care must be taken to see that the joints are sound and 
tight as already described. 

8. Piping and Gas Box. — These should be looked to and cleaned 
about once a month. Impuritiei will settle in any pockets or 
where the course of the gas is not direct. For this reason all 
bent pipes should be avoided as far as possible and when present 
should be examined from time to time. The moisture which 
condenses in the gas box and in the pipe leading from it to the 
engine should be emptied out daily, otherwise it will get into 
the engine and interfere with its working. A drain cock should 
be provided, as at Y, for the purpose of drawing off this moisture. 

9. To Stop the Gas Producer. — The gas cock on the engine 
should be shut and the waste cock / opened so as to allow the 
remaining gas to escape. The taps C and G and the cock H 
must then be closed and the ash door opened a few inches so as 
to allow the fire to continue burning. 

10. To start the Apparatus again after a Temporary Stoppage. — 
The fire and ash doors should be opened to clean the fire, any 
cinders or clinker should be removed without disturbing the fire 
as far as this is possible, the doors should then be closed, the 
cock B opened and the fan started until the fire is again in 
good condition. Anthracite must then be added until a good 
quality of gas is obtained when the engine may be started up to 
work. When the stoppage is only temporary, the scrubber 
and gas box will probably be full of good gas when it takes place 



THE GAS PRODUCER 207 

and it is therefore better to test the fresh gas, made at restarting, 
by means of a test cock placed at Z rather than to test it at Q. 
When good gas is obtained at Z the cock J can be closed and the 
gas then sent through the scrubber and gas box to the engine. 
By following this plan the good gas remaining in the scrubber 
and gas box when the plant was stopped will be utilized, instead 
of being blown away to waste as might otherwise have been the 
case. 

Caution. — The regulation of the supply of water to the coke 
scrubber is important. If the supply be too small, steam will 
be formed in the scrubber, the gas will not be properly cleaned, 
and the quality of the gas will deteriorate. If the supply be too 
great, the water seal of the gas pipe will be too deep and the 
engine will not be able to suck the gas through the producer. 
The coal should not be too large or of unequal size or the air 
spaces between the various pieces will be too great. The guiding 
principle in this is to have a mass of fuel in the producer 
which is as homogeneous as possible without being solid. 
Where coke is used as the fuel a sawdust scrubber is required 
between the coke scrubber and the gas box. When a gas plant 
has been designed for anthracite, other modifications may be 
necessary if it is decided to change from anthracite coal to coke. 



CHAPTER VII 

Blast-Furnace and Coke-Oven Gases 

Thermal Value — Cleaning the Gas — Utilization of the 
Surplus Power. 

94. The Production of Waste Power from Blast-Furnace 
and Coke-Oven Gases. — The idea of using blast-furnace and 
coke-oven gases in gas engines is a relatively modern idea, 
and the extent to which it may be put into force in any 
country depends chiefly upon that country's output in 
pig-iron. The following figures show the output in pig-iron 
in metric tons for the three chief countries concerned — 





1905. 


1906. 


U.S.A 

Germany 

Great Britain 


23,340,258 

10,987,623 

9,746,221 


25,712,106 
12,478,267 
10,311,778 



The gas that issues from blast furnaces is rich in 
carbon-monoxide and poor in hydrogen, and has a 
calorific power of about 90 B.T.U. per cu. ft. : whereas 
the gas from coke ovens is extremely rich in hydrogen 
and may have a calorific value as high as 500 B.T.U. 
per cu. ft. The former is the easier to deal with as it 
comes in steadier quantities, and with the small quantity 
of hydrogen which it contains, pre-ignitions are not likely to 
occur. Consequently it is safe to raise the compression to a 
much higher point (180 lb. per sq. inch or more) than would 
otherwise be safe, and the engine is thereby rendered of 
higher thermal efficiency. Both gases require cleaning so 
far as dust is concerned. 



BLAST-FURNACE AND COKE-OVEN GASES 209 

95. Blast Furnace Gases. — The idea of burning blast- 
furnace gases directly in gas engines instead of under 
steam boilers, as had previously been done, was first 
put into practice about the year 1894, nearly simul- 
taneously in Great Britain, Germany and Belgium. The 
pioneers, prominent among whom was the late Mr. B. H. 
Thwaite, experimented with small engines and, as satisfactory 
results were obtained, it was soon desired to increase the 
scale of operation. In Germany great progress has now 
been made and recently a number of large plants have been 
put in in this country and in the U.S.A. 

The calculation as to the power available in this way in 
Great Britain may be made in the following manner. The 
pig-iron output for 1906 (for example) was, in round figures, 

10,000,000 tons, 

and it is well established that the residual gases from 
blast furnaces in Great Britain as well as on the Continent 
and in America, are capable when used in internal combus- 
tion engines of yielding about 27 h.p. per ton of pig-iron 
per day (the figures given by various engineers are as 
follows : Greiner, 20 ; Bryan Donkin, 28 ; Max Rotter, 25 ; 
Thompson, 20 ; Rossi, 30 to 35). It follows that the whole 
output would be about 

10,000,000 _ w „/>/w*u 

' K — x 27=740,000 h.p., 

365 

of which at present the greater part is going to waste. 

The corresponding h.p. for the 12,500,000 tons of output 
in Germany would be 930,000 h.p., which agrees generally 
with Dr. Hoffmann's estimate of 1,000,000 h.p. 

It is confidently calculated that in those countries where 
this development is in progress a saving of several shillings 
per ton will be made in the cost of producing iron. Several 
German firms, notably, have already found very favourable 
financial results to accrue. 

Professor H. Hubert remarks * that in Belgium the honour 
of being first in the field belongs to Messrs. Bailly and Krafl , 

* Iron and Stool Institute, 100G. 



210 THE INTERNAL COMBUSTION ENGINE 



of the Cockerill Co. The patent taken out by the Company 
for this new application was dated May 15, 1895, and the 
first trials were made at the end of that year. They were 
made with a Simplex engine of 8 h.p. in which the clear- 
ance space had been reduced in order to increase the com- 
pression and to facilitate the ignition of the mixture. The 
gas cleaning was imperfect, and was carried out simply 
by passing it through two scrubbers four metres high. The 
engine is stated to have displayed perfect elasticity, and 
adapted itself to the variations of composition, pressure and 
temperature of the gases. 

The following interesting table is taken from Professor 
Hubert's paper — 



Engine. 



8 h.p. engine 

200 h.p. engine (single cylinder, 
single acting, constant ad- 
mission) ' . 

600 h.p. engine (as above) 

200 h.p. engine (as above, ex- 
cept for variable admission) . 

1,400 h.p. engine (double-acting 
tandem, variable admission). 



Date 

of 
Trials. 



1896 

1898 
1900 

1901 

1906 



Power. 



I.H.P. B.H.P. 



5-26 

213-9 

825-8 

246-9 
1,755 



181-82 
670-0 

215-3 

1,582 



Calories 

used 

per 

I.H.P. 

Hour. 



4,030 

2,775 
2,520 

2,766 

2,129 



Ther- 
mal 
Effi- 
ciency. 



per cent. 

15-77 



22-9 
25-2 

230 

29-8 



96. Coke-Oven Gases. — Coke-oven gases are much richer 
in hydrogen than blast-furnace gases, and they are therefore 
much more liable to pre-ignitions. To avoid this danger, the 
compression is not taken so high, although this precaution 
unfortunately has also the effect of tending to reduce 
efficiency. On the other hand their thermal value is far 
higher, often more than five times as high. To illustrate 
this, the following typical figures are given 
B.F. gas : — 24 J per cent, of CO ; 62 per cent, of N 2 ; 1 J per 

cent, of H 2 ; Calorific value 86 B.T.U. per cu. ft. 
C.Oven gas : — 50 per cent, of H 2 ; 40 per cent, of CH 4 ; 
Calorific value 560 B.T.U. per cu. ft. 

To cajculate the possible output obtainable from coke- 



BLAST-FURNACE AND COKE-OVEN GASES 211 

oven gases in this country is not difficult. Taking the 1906 
output of pig-iron as 10,000,000 tons, the consumption of 
hard coke may be put as about 1 1,000,000 tons. To produce 
this quantity of coke about 15,000,000 tons of coal would be 
required, which on coking would give off about one-fifth of 
its weight in the form of gas, corresponding to about 
500,000,000 cubic feet of gas per day. Assuming that a 
quarter of this is available as a surplus for use in gas en- 
gines, and that it is of the thermal value of 500 B.T.U. per 
cu. ft., the corresponding thermal energy is easily calculated. 
If the gas engines used have a thermal efficiency of 30 per 
cent., the following h.p. would be available : — 

500 778 

i x 500,000,000x — — x _ — x(J-30=306,000 h.p., 
4 24 x 60 33,000 * 

or in round figures 300,000 h.p. This is an estimate for the 
English output. Dr. Hoffmann has estimated the German 
output as from 550,000 to 600,000 h.p. Not a little enter- 
prise has been shown in Germany in harnessing this source 
of power, and action is being taken in this country to the 
same end. 

The proportion of one quarter, used in the above calcula- 
tion * as to the fraction of the gas available for the production 
of this surplus power, depends upon chemical problems, but 
it has recently been found that by raising the temperature 
of the air entering the ovens to 1,000 or 1,100° C. by means 
of regenerators, only 45 to 55 per cent, of the total quantity 
of gas evolved from the fuel is required for the work of heat- 
ing the ovens, so that practically half the gas would in that 
case be available for the production of power in gas engines. 
This idea has been worked out by Mr. Koppers, and at the 
Anna Colliery of the Eschweiler Mining Co., near Aix-la- 
Chapelle, there are reported to be six batteries of Koppers 
regenerator ovens, with a power station designed for the 
production of 16,000 h.p. from the surplus gas. 

* M. Loon Groiner gives the following approximate rules for the 
amount of surplus power available for use : — (a) with blast furnaces. 
the continuously available h.p. is equal to the number of tons of iron 
made per month ; (b) with by-product recovery ovens, the continu- 
ously available h.p. is equal to the number of tons of coke made per 
woek. 



212 THE INTERNAL COMBUSTION ENGINE 

It is on record * that at the Wath Main Colliery, Wath- 
upon-Dearne, Rotherham, an installation of 30 Hiiessener 
patent by-product coke ovens, erected by the Coal Distillation 
Co. of Middlesbrough — representing the Actien Gesellschaft 
fuer Kohlendestillation — has been put in. The plant is 
to produce 800 tons of blast-furnace coke per week, and there 
is to be available sufficient surplus gas and surplus waste 
heat to produce 300 h.p. of electricity from the 30 ovens, 
in addition to meeting the requirements for power for coal 
grinding, elevating, and by-product plants. There are other 
instances of similar enterprise whereby English firms, on 
discarding the old " beehive " type of oven, have been able to 
obtain large quantities of surplus power. Of course there 
are other by-products besides power produced from coke 
ovens, such as sulphate of ammonia, coal-tar and benzole. 

97. The Shelton Iron Works have some Koerting 
Engines working on coke-oven gases, and it has been 
found f that when some coals are used a calorific value of 
over 600 B.T.U. per cu. ft. is obtained, although 400 is more 
common. In no case, however, is the quality constant 
during the whole period of coking. It usually decreases 
from about 450 to 350 during the operation. The gas passes 
through scrubbers where the ammonium sulphate and other 
by-products are collected and most of the tar removed. 
The gas then is divided into two almost equal parts, one half 
going to heat the coke ovens, and the rest to the production 
of power. As the gas contains much hydrogen, naphtha- 
lene, and other highly inflammable bodies, it is liable to 
pre-ignitions, and the compression is kept down to 100 lb. per 
sq. inch, instead of the 140 lb. per sq. inch, which would 
otherwise be customary. The mean pressure works out at 
about 75 lb. per sq. inch. On testing a new variety of fuel 
the following results were obtained : Thermal value of 
gas 381 B.T.U. per cu. ft., engines developed 1 h.p. per 
hour per 22 cu. ft. at full load, or a thermal efficiency of 

' =0305, which seems very high. The analysis 

22x381x778 J 6 J 

* T.E.S., April 17, 1907. 

| Engineering, February 15, 1907- 



BLAST-FURNACE AND COKE-OVEN GASES 213 



of the gas was 

C0 2 • • 3 ' 55 per cent. 

Olefines, etc 5 18 

2 1-59 

Methane 27-82 

H 2 54-33 

N 2 316 

According to some figures in The Engineer, of 22 installa- 
tions in Germany with a total output of 13,000 h.p. from 
engines working on coke-oven gas, no less than eleven, or 
half of them, do not find it necessary to clean the gas. 
One of them was stated to be using gas with 2 per 
cent, of sulphur without injurious effect on the iron. 

98. Cleaning the Gas.— It has been found that the most 
effective way of cleaning the gas is by the action of a water 
fed fan. The gas passes through a centrifugal fan which 




Fig. GO. — Theisen Gas Washer — Section. 



causes the heavy particles of dust to fly outwards, and at the 
same time water is fed into the fan and broken up by the 
same centrifugal action. This water catches up the dust 
particles and passes with them to a sump. Perhaps the best 
known gas cleaner of tins type is the Theisen Patent On- 



214 THE INTERNAL COMBUSTION ENGINE 

trifugal Central-flow Gas Washer, made by Messrs. Richard- 
sons, Westgarth and Co. It is illustrated in Figs. 60 and 61. 
The Theisen machines are specially adapted for cleaning 
gas, and particularly blast-furnace gas, for use in gas engines 
and where a high degree of purity is required. When very 
hot and dirty gas has to be treated, it is considered advisable 
to instal a preliminary saturator before the washer, where 
the gas may be cooled and the heavier dust removed. In 
this way not only is the volume of gas to be cleaned reduced, 
but less water is required in the washer itself, and conse- 
quently less power is absorbed. The makers claim that the 



Gas Outlet 




CulverlJ 
Fig. 61. — Theisen Gas Washer — End Elevation. 



power taken to drive the cleaner does not exceed 2 per cent, 
of the maximum power which could be generated in gas 
engines from the gas cleaned. The quantity of water 
required by the Theisen apparatus varies with the tem- 
perature of the gas and the amount of dust therein, and in 
addition with the degree of cleaning necessary. With hot and 
dirty gas it sometimes happens that as much as 1 litre of 
water is required per cubic metre (or 1,000 litres) of gas 
cleaned, but usually half this quantity will suffice. Of 
course the water can be used again and again, if the dust 
be allowed to settle out of it. The makers have published 
the following table shoAving results of trials — 



216 THE INTERNAL COMBUSTION ENGINE 

The amount of dust in the gas can be measured very 
easily. It is only necessary to pass the gas through a 
filter consisting of a glass tube filled with absorbent 
cotton. The quantity of gas passed is measured in a 
meter, and the cotton is weighed before and after. The 
method is stated to give accurate results if the cotton is 
evenly packed along the tube and is not hydroscopic. In 
any case the cotton should be dried before and after in a 
desiccator, and weighed from time to time to check whether 
any moisture is held in it. 

In America,* peculiar difficulties are experienced, owing to 
the character of the ores used. The Mesati ores are stated to 
be especially troublesome, owing to their friable nature. With 
every disturbance in the furnace great quantities of dust are 
evolved, which often pass the entire cleaning plant unless un- 
usual precautions are taken. The suspended matter consists 
largely of ore dust together with some additional matter 
carried over from the other constituents of the furnace 
charge. At Bessemer, however, a cleaning plant has been 
put down which has cleaned the gas as low as 0*1 grains per 
cubic foot, which is considerably cleaner than the surround- 
ing air in that particular locality. In practice, however, the 
engines work quite well with ten times this amount of dust. 

99. Utilization of Surplus Power. — The utilization of the 
power derivable from the waste gases of blast furnaces and 
coke ovens is a problem in itself. The solution of this 
problem must depend upon the extent to which local demand 
for power exists, or can be created. It is only necessary to 
think of such electro-metallurgical processes as the manufac- 
ture of aluminium to bring to mind the possibility of the crea- 
tion of huge demands for current under the happiest condi- 
tions in respect of load factor. For the transmission of such 
power for any distance less than half a dozen miles, it would 
probably be most economical to use pipe lines to convey 
the gases, but for longer distances electrical transmission 
would be the obvious method to adopt. Another industry 
that might be served is the manufacture of calcium carbide. 
Carbide is not now being manufactured in bulk in this 

* T.E.S., July 17, 1907. 



BLAST-FURNACE AND COKE-OVEN GASES 217 

country, owing to the lack of cheap power. Abroad, engineers 
have the advantage of extraordinarily cheap water power — 
as low, according to Professor S. P. Thompson, as ^6 P ar ^ 
of a penny per h. p. -hour — and it is clear therefore that unless 
some very cheap source of power is rendered available here 
also it will not be possible for this country to produce its 
own carbide. Calcium carbide, in its purest form, is used 
for the production of acetylene for lighting purposes, but a 
less pure and cheaper kind can be used in the preparation 
of chemical manure, for which the demand is on an alto- 
gether larger scale. 

Lime and coke when heated together to a temperature of 
2,000-3,000° C. produce calcium carbide, combining in 
accordance with the following chemical formula — 
CaO+3C=CaC 2 +CO. 

This reaction is carried out in an electric furnace worked 
either by direct or alternating current, although as the 
latter allows of a higher voltage transmission and 
simple transformation, it is usually preferred. It is a 
high temperature reaction and not an electrolytic one, 
thus permitting either type of current to be used. In the 
above equation the CO passes away as a by-product, and 
carries with it one-third of the carbon used. This gas 
might of course be collected and its thermal value used 
say for the heating up of the charge of lime and coke, for 
the earlier part of the great temperature range necessary. 
The amount of current needed to produce 1 ton of calcium 
carbide is about -§■ h. p. -years. Mr. Bertram Blount in his 
Practical Electro-Chemistry remarks : — " The surplus gas 
(from coke ovens and blast furnaces) can be used with econ- 
omy in large gas engines of 500 or 1,000 h.p., and energy thus 
obtained almost as cheaply as from a water-power. For 
example, at an inclusive cost of ^d. per h.p. -hour, which 
is by no means unattainable, the price per h.p. -year is £3 1 3s., 
a figure which approaches that of a moderately cheap water- 
power. The real obstacle to the general utilization of such 
power is not its cost, but the somewhat restricted market 
for carbide, causing it to be readily swamped by any groat 
increase of supply; even with that restriction, however, the 



218 THE INTERNAL COMBUSTION ENGINE 

manufacturer having cheap coke and lime in an industrial 
centre, will stand at least as good a chance as his rival with 
slightly cheaper power, but away from such supplies." 

100. Owing to the discovery that calcium carbide could 
be used in the preparation of an excellent chemical manure, 
the possibility has been opened up of an enormous demand 
for this product, thus affording a suitable purpose to which 
large quantities of electric power could well be devoted. 
Such an enlargement of the calcium carbide market might 
not be altogether welcome to present manufacturers of the 
carbide, as the new product, not being used in the production 
of acetylene gas for lighting need not be so pure. A heavy 
demand for the less pure carbide might therefore lead to 
difficulty in obtaining small supplies of a purer kind, as it 
would hardly be worth while undertaking it. Or even if un- 
dertaken, the cost of such carbide might actually be greater 
with the increase of output than it is now. Probably if 
the bulk of the output were of a different quality it would not 
be feasible commercially to produce raw carbide in so pure 
a state, but this would not prevent the impurer carbide being 
purified by subsequent treatment in such quantities as the 
acetylene demand might necessitate. Even if chemical 
difficulties present themselves in the purification of the 
carbide when made, there is no reason to suppose that the 
ingenuity of chemists will be unable to circumvent those 
obstacles as soon as it is necessary for them to be dealt with. 
Present-day manufacturers hold to prevention being better 
than cure, and would far rather see that purer raw materials 
(coke and lime) were used ; but if a big agricultural demand 
should arise, it is not to be expected that subsequent modes 
of manufacture would be controlled entirely with a view to 
the smaller market. The virtue of calcium carbide from 
the agricultural point of view lies in the fact that it can be 
converted into calcium cyanamide, which can be directly 
applied to land as a fertilizer, and that when so employed it 
is of great value and efficacy. The cyanamide can be ob- 
tained direct from the carbide by fusing the latter in a stream 
of nitrogen. Or if preferred, the process may be shortened 
by admitting nitrogen to the electric furnace in which the 



BLAST-FURNACE AND COKE-OVEN GASES 219 

lime and coke are being fused. As the author pointed out 
some years ago * — " The question for English engineers 
is, whether it is commercially possible (scientifically it 
certainly is), for a carbide industry to be set up in this 
country, drawing its electrical power from the waste gases 
of the English iron districts, and relying on a sufficient econ- 
omy in regard to other items of cost, such as capital charges, 
labour, lime, coke, and carbon electrodes to enable carbide 
to be produced and sold at a profit at a rate not higher than 
the £13 or so a ton for which it can now be obtained in the 
market. It appears to the writer that this recovery of a 
British industry is well worth attempting, especially in view 
of the possible creation of an agricultural demand so keen 
that the supply would of necessity lag behind the demand 
for some years, thus providing a condition which would 
be most favourable to the initiation of the enterprise. What 
the saving on freight would be if the carbide were produced 
in this country instead of abroad would depend largely upon 
the relative views taken by railway and steamship com- 
panies as to its ' danger ' in transit, but it is fairly obvious 
that the balance of the advantage, whatever it may amount 
to, should, in most localities, lie with the English producer." 
As above quoted, Mr. Blount put the figure at which power 
could be obtained from waste gases at as low a figure as 
£3 13s. per h. p. -year, but Mr. B. H. Thwaite, in his Iron and 
Steel Institute paper in 1 907, put the cost at an even lower 
figure than that. His figure was £3 6s. H. per K.W.-year, 
corresponding to less than £2 10s. per h. p. -year, and Mr. H. 
Greiner, who followed in the discussion, stated that his ex- 
perience showed a cost of about 80 francs per K.W.-year, 
equivalent to about £2 8s. per h.p.-year. Mr. Thwaite's plan 
was to collect the waste gases from all the furnaces of the dis- 
trict in which iron was being made, and utilize it in gas en- 
gines for the production of electric power which would be dis- 
tributed to customers. The first call on the former would be 
for the demands of the steel works concerned, and the bal- 
ance could be sold to any one else who might want power. 
This plan would have the merits of decreasing capital charges 
* T.E.S., October 3, 100(5. 



220 THE INTERNAL COMBUSTION ENGINE 

for plant, and of increasing the load factor. According to 
Mr. Greiner about 7,000 e.h.p. was then being generated at 
the Cockerill works from coke-oven and blast-furnaces gases, 
and it was intended to increase this output very largely. 
In the United States the utilization of blast-furnace gases in 
gas engines practically began at the Edgar Thompson works, 
of the Carnegie Steel Co. at Bessemer near Pittsburgh, by the 
installation of several 3,000 h.p. Westinghouse engines. 
The experiment being successful the United States Steel 
Corporation decided to increase the capacity of the gas 
engine plant to 50,000 h.p. It is reported that very little 
trouble has been found in the working of the plant so far 
installed, despite the absence of experimental data or ex- 
perience of continuous working. 



SECTION III 
OIL AND PETROL ENGINES 



CHAPTER VIII 

Oil and Petrol Engines 

Fuels — Slow-Speed Oil Engines — Diesel Engine — Petrol 
Engines — Carburettors — Governors — Theory of Jet 
Carburettors — Ignition. 

101. Fuels. — Internal combustion engines are of two 
classes : ( 1 ) those that work with gases for their explosive 
medium, and (2) those that use vapours of liquid hydrocarbons 
such as oils. The former class has been dealt with in the 
preceding chapters so far as everything except methods of 
ignition is concerned — and ignition being similar in both 
classes does not need to be dealt with in two parts. Oil 
and petrol engines, as those in class (2) are generally 
named, are of practically the same design as gas engines so 
far as cylinders, pistons, valves, etc., are concerned, and the 
difference between them mainly relates to the mechanism 
for dealing with the fuel used. A gas engine does not need 
any carburettor, whereas in the oil or petrol engine it is one 
of the most important and most sensitive parts. 

Fuels for oil and petrol engines may be divided into 

(1) heavy oils and (2) spirits. Heavy oils include everything 

from crude Borneo oil (looking like thin treacle) to paraffin 

(such as is commonly used in oil lamps). The spirits used 

are chiefly petrol, benzol and alcohol, but a great number 

of other variations have been suggested by ingenious persons. 

The reason why it is so easy to find new petroleum spirits 

which are usable in internal combustion engines is that 

each change in the temperature range over which the oil is 

distilled practically produces a new substance. The names 

of the various spirits are therefore numerous and not a little 

confusing. There is the additional complication that in the 

United States what would here be called petrol is known as 

gasolene, and for paraffin they use the word kerosene. 

223 



224 THE INTERNAL COMBUSTION ENGINE 

The following are the calorific values of the various fuels 
named — 



Fuel. 



Calorific Value (higher value) 



In B.T.U. per lb. In ft. -lb. per lb of Fuel 



Absolute alcohol ... 12,600 9,800,000 
Methyl alcohol (with den- 
sity— 0-82) .... 11.300 8,800,000 

I from 20.300 ffrom 15.800,000 

I to 19.300 t to 15,000.000 

Paraffin 23,100 18,000,000 

Denatured alcohol (methyl- 
ated spirits) density =0-83 11.000 8,600,000 



* Petrol with density =0-722 



102. According to Professor Vivian B. Lewes, as the crude 
oil comes from the well it is a mixture of many hydro- 
carbons and varies considerably both as regards its physical 
and chemical properties, according to the source from which 
it was obtained ; the American and Russian oils upon 
which most scientific work has been done differ widely in 
their chemical composition, although much alike in their 
physical properties. 

Pennsylvanian petroleum consists of a mixture of hydro- 
carbons of the C n H 2n+2 group in which n may be anything 
from 1 to, say, 30, and the boiling point rises gradually from 
0° C. for C 4 H 10 to 280° C. for C 16 H 34 . Hexane (C 6 H 14 ), 
which is practically petrol, has a boiling point of 69° C. and 
a density of 0-664. 

The density of the oil on leaving the well is about 0*84 
to 0-90, and the output for the whole world is in round 
figures 20,000,000 tons. This total will be observed to be 
a very small one when compared with the output of coal, 
which is about forty times as great. Unless therefore fresh 
supplies of oil are discovered it is no use to hope to replace 
solid by liquid fuel, although the higher calorific value 

* Professor Hopkinson found the calorific value of a sample 
of Pratt's motor spirit (density =0-715) to be 17,500 B.T.U. per 
pound on the lower value (i.e. assuming steam formed during com- 
bustion not to be condensed). 



OIL AND PETROL ENGINES 



225 



of the latter might make the change desirable. 


The sources 


of the supply of petroleum in 1907 were as follows — 


Russia 28*0 per cent. 


U.S.A 596 


,, 


Dutch Indies . . . . 33 


?? 


Roumania 2*9 


55 


Galicia 2*5 


5? 


India . . . . . . 1'9 


}) 


Other countries . . . . 18 


55 


1000 


53 



It will be seen that the first two countries produced 
nearly 90 per cent, of the whole. 

103. Coal Tar Products. — Useful fuels are found among 
the by-products of gas works and, as will be explained 
later, some of them have been used successfully in internal 
combustion engines. Mr. O 'Gorman has summarized the 
coal tar by-products as follows — 

1. Ammoniacal liquor. 

2. First light oils. 1 „ , ,,, 

3. Second light oils. } Crude naphtha. 

4. Carbolic oils. 

5. Anthracite oils. 

2 and 3 can be broken up by distillation into : — ■ 

(a) Benzole. 

(b) Solvent naphtha. 

(c) Illuminating oils. 
Benzole in turn distils to — 

(i) " 90 per cent, benzol " (so called because 90 per 
cent, distil out before 100° C). 

(ii) " 50 per cent, benzol " (so called because 50 per 
cent, distil out before 100° C). 

(iii) Solvent naphtha. 

104. Ideal Conditions. — The ideal fuel would be one which 
behaved uniformly in every part when subjected to an 
increasing temperature, one, for instance, which would begin 
to distil at a temperature quite close to that at which 
distillation ended. That the opposite condition to this is 



226 THE INTERNAL COMBSUTION ENGINE 

an undesirable one will be understood by reflecting that if 
the temperature range of distillation be great there is a 
considerable chance that in the engine this fuel might be 
subjected to selective action such as would leave the heavier 
parts of the fuel as a deposit in the cylinder with consequent 
loss of horse-power and " gumming-up " of piston and valves. 

The ideal fuel must be clean and easy to handle without 
danger. It should likewise be cheap. It is here that the 
disadvantages of petrol are so marked. It is both costly 
and dangerous, but is otherwise a good fuel because its 
range of distillation is small. The avoidance of danger 
requires the absence of low flash point, but on the other hand 
a low flash point fuel is good for starting the engine as there 
is then no need to heat the engine or carburettor first. If 
a low flash fuel is avoided the engine is less easy to start. 
On the whole, the ideal conditions are seen to be mutually 
conflicting. 

Petroleum derivatives (i.e. everything from crude oil to 
petrol) are commonly so far lacking in uniformity of com- 
position that the flashing point is governed by some small 
amount of lighter spirit which is present (even 1 or 2 per 
cent, will fix the flashing point) so that the measurement of 
this point does not tell one much as to the real nature of the 
bulk of the fuel. It does, it is true, tell one that the carburet- 
tor will need so much the less external heat to be applied, 
but of the closeness of the points over which the oil will distil 
it tells nothing. A medium heavy petrol (density 0*760) 
will distil completely between 60° C. and 150° C, a narrow 
range of but 90° C. Some oils, however, have a range of 
hundreds of degrees and would therefore be unsuitable 
for use in an internal combustion engine.* 

The following practical test results have been obtained 
in the States on three exactly similar Maxwell cars driven 
by gasolene, kerosene and alcohol respectively over 249 
miles of roads which were snow-covered in places to the 
depth of 10 inches — 

* The best petrol has a specific gravity at 15°C of 0-715 to 0*730, 
and yields 63 per cent, of constituents (by volume) at and below 
100°C, and 90 per cent, at and below 120°C. 



OIL AND PETROL ENGINES 



227 





Cost 


Total 


Cost of 


Cost 


Cost 


Miles 


Fuel. 


per 


consump- 
tion in 
Gallons. 


Fuel 


per 


per 


per 




Gallon. 


per Car. 


Mile. 


Ton-mile 


Gallon. 




$ 




$ 


$ 


$ 




Gasolene (or 














petrol) . 


0-20 


24-75 


4-95 


019 


00169 


101 


Kerosene (or 














paraffin) 


013 


33-75 


4-39 


017 


00139 


7-4 


Alcohol .' . 


0-37 


40-75 


1507 


0-60 


0-448 


6, 



105. These figures show that alcohol is not — in the United 
States at least — a cheap fuel and that kerosene is. The 
Fuels Committee of the Motor Union, however, appear to 
have considered alcohol as a possible alternative and rival 
to petrol, and their 1907 Report dealt largely with this 
possibility. The following extracts from the Report are 
given — 

Most readers of this Report are familiar with the properties of 
petrol as a fuel, but they have very little idea of the great advantages 
of alcohol, having probably only heard of certain objections more 
or less imaginary, such as corrosion, and it has, therefore, been 
thought desirable to add the following summary of the properties 
of alcohol, comparing them with those of petrol : 

(1) Safety, (2) thermal efficiency, (3) calorific value, (4) practical 
limit of compression, (5) complete combustion, (6) propagation of 
the flame, (7) smell, and (8) flexibility. 

(1) Safety. — In the first place, in case of possible conflagration, 
alcohol can be extinguished by water, whereas petrol is only scat- 
tered under similar circumstances and the area of conflagration 
increased. In the second place, and even more important, the flash 
point is considerably higher, being 60° Cent, compared with petrol, 
which may be taken as anything down to 10° Cent, below freezing 
point. This enables the alcohol to be carried and stored with safety 
under conditions where petrol would not be permitted. This 
further very much reduces the cost of freight and insurance. 

(2) Thermal Efficiency. — Owing to less air being required and a 
consequent reduction in the amount of inert gas, the thermal 
efficiency of alcohol is as high as 35 per cent., as against something 
below 20 per cent, in the case of petrol, and this greatly reduces 
the chances of overheating, besides also reducing the weight of 
cooling water, radiator, etc. 

(3) Calorific Value. — The calorific value of absolute alcohol is 
12,600 B.T.U., that of methyl alcohol with a specific gravity of 
0-820 is 11,300, and alcohol with the addition of 20 per cent, of 
water shows a calorific value of 0,810 ; whereas that of petrol with 
a specific gravity of 0-722 ranges from 20,300 to 19,300 B.T.TJ, 



228 THE INTERNAL COMBUSTION ENGINE 

(4) Practical Limit of Compression. — The practical limit of com- 
pression of alcohol is about 200 lb. per square inch. ; and its explosion 
pressure is therefore considerably higher than that of petrol, the 
practical limit of compression of which — in view of possible pre- 
ignition — is limited to 80 lb. per square inch. 

(5) Complete Combustion. — With alcohol complete combustion 
is more easily attained, owing to the fact that it distils completely 
in its commercial form over a small range of temperature (80-100° 
Cent.), a very accurate degree of carburation thus being maintained. 
In the case of petrol the range of boiling point extends between 
50° Cent, and 150° Cent. ; such a large range of boiling points renders 
accurate carburation at all times more difficult, and makes the 
spirit what is commonly known as stale owing to the evaporation 
of the lighter fractions. Alcohol has not this disadvantage, the 
liquid being practically homogeneous throughout. 

(6) Propagation of Flame. — There is less rapid propagation of 
the name when alcohol is used, which gives a much more uniform 
pressure throughout the stroke than petrol. 

(7) Smell. — With alcohol there is approximately no offensive 
smell in the exhaust, as compared with petrol. 

(8) Flexibility.— Alcohol will explode when mixed with air over 
a wider range than petrol — 4- 13 per cent, alcohol vapour in air 
being combustible, the range in the case of petrol vapour being 
2-5 per cent. ; thus the engine will be much more flexible. 

There are three points, however, on which it is popularly supposed 
that alcohol compares unfavourably with petrol. These are : 
(9) Corrosive effect. 

(10) Starting from cold. 

(11) Vaporization. 

(9) Corrosive Effect. — With regard to alcohol, any corrosive effect 
that may occur is probably due to impurities in the denaturing agent 
present in acetone and methyl alcohol, but these difficulties would 
be overcome if the carburation is such as to give complete 
combustion. Upon this point Dr. W. R. Ormandy writes to the 
committee as follows : 

" My information with regard to the action of the effluent gases 
from motors running on alcohol was obtained from the engineer at 
the Gahrungsversuchsanstallt at Berlin, who reported that engines 
running on pure alcohol, or even on pure alcohol with the German 
denaturant, gave no appreciable corrosion except on such parts of 
the motors as were so cold that condensation took place ; thus the 
silencer was apt to corrode, more so the larger the percentage of 
water in the alcohol employed. As the average amount of water at 
present in German industrial alcohol is 10 per cent., this corrosion 
might become appreciable if the cooling of the cylinder walls was 
too effective. It has been proved, however, that the efficiency of 
alcohol engines is enormously increased by keeping the cylinder walls 
near the temperature of boiling water, and under these conditions 
no condensation and no corrosion obtained." 

(10) Starting from Cold. — As for difficulty in starting from cold, 
it will be probable that alcohol as a fuel will almost always have a 
greater or less quantity of benzol mixed with it, in which case this 



OIL AND PETROL ENGINES 229 

difficulty entirely disappears. Even without the addition of benzol 
there is little doubt that the question of starting from cold will be 
almost entirely overcome by the use of a suitable carburettor. 

(11) Vaporization. — Alcohol requires 5^ per cent, of its total heat 
of combustion to vaporize it, whereas, on the other hand, petrol 
vaporizes without any external assistance. With regard to the 
heat required to vaporize it, it is to be noted that, inasmuch as a 
large amount of the heat produced passes off in the exhaust, this is 
really available for the purpose of vaporization and does not repre- 
sent any thermal loss. 

Other Means of Utilizing Alcohol. — From the previous argument 
it will be seen that, in order to utilize alcohol in an internal com- 
bustion engine, certain modifications in the engine itself become 
necessary, but it is quite reasonable to expect that such alterations 
would be unnecessary if the proportion of tar benzol, acetylene, or 
other hydrocarbon containing a high percentage of carbon were 
mixed with the alcohol. Owing to this high percentage of carbon 
present, the chemical composition of the mixture will be brought 
more nearly to resemble that of the petroleum products. As to 
the most suitable relative proportions, experiment only will deter- 
mine these, but such a fuel as is here suggested has the advantage 
of being a home production, as well as one that could be used without 
material alteration to the engine. 

***** 

It has been stated in evidence that the average price at which 
alcohol can be produced in Germany amounts to Is. a gallon, in- 
cluding the cost of denaturing and Government supervision. It is 
also a fact that in this country the actual cost of manufacturing 
alcohol amounts to ll^d. a gallon (64 overproof, a strength common 
in industrial spirit) — see Report of Departmental Committee on 
Industrial Alcohol. This is produced from beet, potatoes, and 
molasses. Evidence has been given which tends to show that 
alcohol may also be produced from sawdust at a very low cost. 
The lowest figure it is possible to touch in this respect is 3d. per 
gallon when peat is used. Now, owing to the great strictness of the 
Excise authorities in England, the cost of denaturing and expenses 
of supervision bring the total cost of the alcohol up to about 2s. 
per gallon at the present time, and it is therefore evident that 
should the Government see their way to take a wider view of the 
question of alcohol as a fuel for internal combustion engines this 
price of 2s. a gallon could be very materially reduced. If this were 
done, the price could easily be brought to such a figure that it 
would be a very serious competitor with petrol in this respect alone. 

The Government that will recognize this, and will allow untaxed 
alcohol suitably denatured to be used for light, heat, or power, will 
be conferring an immense boon and benefiting a very large propor- 
tion of the population. 

As regards the possible use of benzol the Committee remark : 

What is commonly known as 90 per cent, benzol, can bo utilized 
with perfect success in the engine of a motor car either alone or 
mixed with petrol, or mixed with alcohol. Owing to the high 
percentage of carbon which is found in benzol, and to the low 



230 THE INTERNAL COMBUSTION ENGINE 

percentage of carbon in alcohol, it is evident that a mixture of these 
two liquids more nearly approaches the ordinary hydrocarbon liquid 
fuels to which we are accustomed in its-: chemical composition. 
Benzol will carburate air in the ordinary way when an ordinary 
petrol carburettor is used, but its specific gravity is very much 
higher than that of petrol, viz. 0-883, which may necessitate an 
adjustment of the float to prevent the benzol standing too low in 
the jet of the carburettor. Crude benzol inevitably contains a 
certain amount of foreign matter in combination with sulphur, 
which imparts to it an unpleasant smell in the liquid state. Owing 
to its comparatively low price, however, it might p a y to have 
benzol still further treated after washing in order to remove these 
impurities, which could be done for the expense of about Id. per 
gallon. At the present time benzol cannot be obtained in very 
large quantities, as the number of recovery plants in this country 
is not very large. As benzol is a home production, its use should 
be encouraged, and particularly at this present time when the differ- 
ence between the prices of petrol and benzol is very small. 

^Mixtures of benzol and alcohol have been tried in a desultory 
manner on the Continent, but in this country nothing has been done 
upon an extensive scale. The possibilities for the successful use 
of such a mixture are very great, and both these fuels are capable 
of manufacture in this country in very large quantities. Although 
a mixture of benzol and alcohol is in its normal state quite nauseous, 
and would not require a further treatment such as the addition of 
wood naphtha, yet it is possible., at any rate, to partially separate these 
two liquids, the alcohol having an affinity for water. 

106. Sources of Petrol Supply. — The advent of the motor 
car has been the main cause of the increase in the demand 
for petrol, which previously had been regarded as a waste 
product. Petrol is now one of the most valuable com- 
ponents of crude mineral oil. According to Mr. Duckham 
the following are the leading sources of the petroleum 
spirit (or petrol) imported to this country : — 



From 


1904. 

Per cent. 


1905. 
Per cent. 


1906. 
Per cent. 


United States 

Sumatra, East Indies, Borneo 
and Netherlands .... 

Roumania 

Other Countries 


50 

37 

8-2 
4-8 


56 
42 


29-8 

61 4 
8-6 
0-2 




1000 


1000 


1000 



107. Slow-speed Oil Engines. — As an ordinary petrol engine 



OIL AND PETROL ENGINES 231 

which is run on paraffin becomes thereby an " oil engine," 
it is necessary to bring in the variable factor, speed, in 
order to distinguish it from the older and heavier types of 
oil engine. The former runs at, say, 1,000 revolutions per 
minute, and the latter at only a few hundred — quite com- 
monly 200, although there is not the same general constancy 
of speed in this class that there is in the other. Neither is 
essentially different from the other. If a petrol engine is 
imagined as greatly increased in size — say to a cylinder 
diameter of 14 in. — and all parts increased in proportion, 
the safe speed at which the engine will run will have to be 
reduced, because whilst the weight of moving parts goes up 
with the cube of the dimensions, sectional areas of stressed 
metal only increase as the square. It is not possible therefore 
to aim at high speeds without greatly increasing the cost 
of production. The building of such expensive engines is 
frankly put on one side and a cheap engine is built which 
will run at a very slow speed, slower even than in proportion 
to the increase in dimensions would naturally suggest. This 
enables cheaper materials to be used than are employed 
in the construction of petrol engines. The output in horse- 
power is reduced in proportion to the speed, but increased 
as the square of the cylinder dimension, provided that ports, 
etc., are designed of sufficient size to enable the working 
mixture to enter and leave the cylinder without undue 
obstruction. 

As representative of this heavier class of engine the well- 
known Campbell oil engine is selected, and following on to it 
will be given a short description of the Diesel engine which 
works in a special way and is capable in proper hands of 
showing an exceedingly high thermal efficiency. 

108. The Campbell Oil Engine is illustrated in Figs. 02 
63, 64, 65, 66 and 67. Fig. 62 shows the engine to 
be somewhat similar in plan to a horizontal steam engine, 
and the engine parts are generally on that scale. The inlet 
valve G and exhaust valve G are shown in position. The 
latter is worked through a lever H and side rod J by an 
eccentric K driven from the crank-shaft L by spur gearing. 
When the speed exceeds the normal, a centrifugal governor 



232 THE INTERNAL COMBUSTION ENGINE 




Oil and petrol engines 



233 



INDICATOR SCREW 



CYLINDER JACKET 




Fig. 63. — Section through exhaust valve and plug of Campbell Oil Engine. 

5_ 




>' 



Fig. 04. — Campbell Oil Engine, illustrating operation of vaporiser 



234 THE INTERNAL COMBUSTION ENGINE 

pushes down a steel piece N, which engages with a cor- 
responding steel piece on the exhaust lever H, and pre- 
vents the exhaust valve G from closing. When this valve is 
held open no partial vacuum can form in the cylinder during 
the charging stroke of the piston because there is free com- 




Fig. 65. — Section through 
inlet valve and plug of 
Campbell Oil Engine. 



Fig. 67. — Campbell Oil Engine, piston detail. 



munication with the atmosphere through the exhaust 
valve, and consequently no charge of oil and air can be 
drawn into the cylinder. The vaporizer for combining air 
and oil into an explosive mixture is shown in section in 
Fig. 64 and consists of a cast-iron chamber A securely 
bolted to the cylinder and in direct communication with 
the combustion chamber. Into the top of this chamber the 
inlet valve plug B is fitted and this plug contains the seat 
of the inlet valve C (see Fig. 62). The inlet valve C is 



OIL AND PETROL ENGINES 235 

kept closed by a light spring D and only opens during the 
charging stroke of the piston when a partial vacuum is formed 
in the cylinder. Oil is admitted through the annular space 
or groove F and passes through small holes in the valve 
seat and into the vaporizer when the inlet valve leaves its 
seat. Air is admitted through the pipe M and passes 
through the inside of valve plug B carrying the oil with it. 
The ignition tube E is screwed into a boss on the lower 
portion of the chamber A. The tube and the whole of the 
vaporizer is kept hot by an external lamp. During the 
charging stroke of the piston, a partial vacuum is formed 
in the cylinder and the charge of oil and air is drawn through 
the inlet valve, being sprayed during its passage against 
the heated sides of the chamber A and thus vaporized. The 
mixture then passes into the cylinder, is compressed on the 
return stroke of the piston and then fired by the heat from 
the ignition tube. The timing of ignition is left to adjust 
itself, once the correct ignition has been found. Governing 
is, of course, by missing strokes and not by throttling. 

109. The Hornsby Type of Vaporizer is also worth studying. 
This type of vaporizer is shown in Fig. 68, and to show 
how the vaporizer is fitted in place the diagram includes the 
cylinder also. When it is desired to start the engine a 
lamp is placed under the vaporizer chamber until the latter 
is at a sufficient temperature to ignite the oil which is 
pumped into it. This lamp is withdrawn once the engine 
is started as the heat of explosion is sufficient to keep the 
temperature up to the requisite point. The oil tank is 
under the engine and from it the oil is forced by a small 
pump into the vaporizer just at the moment when the piston 
is starting on its out-stroke and is drawing in the air neces- 
sary to combustion. The supply of oil is controlled by the 
governor in the following way. The oil passes through a 
valve-box with two valves, one of which leads to the vapor- 
izer and the other leads to an overflow from which the oil 
can flow back to the tank. If the speed rises beyond the 
required point the governor opens this latter valve and the 
quantity of oil getting into the vaporizer is therefore 
reduced. On the return stroke of the piston the mixture 



236 THE INTERNAL COMBUSTION ENGINE 




is compressed and some of it forced back into the hot 
vaporizer where the temperature is so high that ignition 



OIL AND PETROL ENGINES 



237 



occurs and a working stroke is therefore made by the piston. 
The vaporizer chamber can, of course, be taken out and 
cleaned when desired. It is found, however, that even when 
working on quite heavy unpurified oils very occasional 
cleaning will suffice. 

110. The Diesel Engine differs from the above type of 
slow-speed oil engine in that the oil is admitted gradually 
during the explosion stroke instead of during the suction 
stroke. It works on the Otto or four-cycle principle just 
as the Campbell engine does, but the details differ. Thus 



K -sP^^F 






^ 






ijlg£g§*\ 




, — . — "^^^^^ 





Fig. 69. — Three-cylinder " Mirrlees Diesel " Oil Engine coupled direct to 
90 K.W. Generator. For Birkdale District Electric Supply Co. 

in the Diesel engine {see Figs. 69, 70 and 71) air alone 
is taken in during the suction stroke. Air alone is com- 
pressed to 35 or 40 atmospheres and to a temperature of 
1,000° F. (a dull red heat) or more. Then on the ensuing 
outward stroke oil is sprayed into the cylinder at such a 
rate as to produce during the first part of the stroke a 
nearly uniform pressure of about 500 lb. per square inch. 
At a given point in that stroke the fuel supply is cut off and 
expansion takes place. Then follows the usual scavenging- 
stroke. The speed for a 240 h.p. engine is about 160 revo- 
lutions per minute. It is claimed that the fuel consumption 



238 THE INTERNAL COMBUSTION ENGINE 

need not be more than 0*40 lb. of oil per b.h.p.-hour. If 

the oil have a calorific power of 15,000,000 ft. -lb. per pound, 

this is equivalent to the engine yielding 1,980,000 ft. -lb. 

(being one b.h.p.-hour) for every 0-40x15,000,000 ft.-lb. 

,.,.,.. m . , 1,980,000 , 

put into it, giving an efficiency of — - or almost 

J 6,000,000 

exactly one-third or 33 per cent. This is a very high effi- 



a-Air Suction Valve 
br Exhaust Valve 
c.-Fuel Injecting Valve 
dr Starting Valve 
erStarting Lever 




Fig. 



70. — Sectional view of Diesel Oil Engine (Mirrlees, Watson and Co.). 
Note position of inlet valves. See also Fig. 71. 



ciency, and the reason why it can be obtained is mainly on 

account of the high compression employed, viz. 35 or 40 

as against 5 or 6 as commonly used on other engines. The 

Diesel engine cycle aims at compliance with the principle 

of taking in all its heat at constant pressure and the possible 

/ 1 \ y ~ 1 
efficiency given by the formula >y = l — f — J would 

when r==40 be no less than 0*78 as compared with a cor- 






OIL AND PETKOL ENGINES 



239 



responding figure of 0-48 when r = 5. This is enough to 
show that so considerable a rise in compression ratio might 



Top position 



S arting Vessel 
Pressure 




Fig. 71. — Side view of Engine shown in Fig. 70. 

be expected to lead to a high thermal efficiency, and engine 
tests have confirmed this. 

111. The Thornycroft marine engine can be operated 
with either petrol or paraffin. It is, of course, easier to 
work a marine engine on paraffin than a land one, as in the 
former the starting torque required is very slight and the 
speed at which the engine runs is much more even. There 
are, in short, no hills to climb. 

The Thornycroft engine is illustrated in Figs. 72 and 
73, and the following description will help to elucidate 
them. 

In the first place it will be noticed that the engine is 
essentially a marine one, the bearing arms being cast on 



240 THE INTERNAL COMBUSTION ENGINE 

the bottom half of the bed-plate, and large doors being 
fitted in the upper half to enable adjustments to be made 
to the bearings, etc. It will also be noticed that the engine 
is substantially built and suitable for heavy continuous 
running at full power. 

The cylinders M are cast in pairs with large water-jackets 




Fig. 72. 



General Arrangement of Thornycroft 6" X 
Paraffin Engine — End view. 



Marine Petrol or 



N surrounding them ; these water-jackets extend sufficiently 
far down to enable the working parts of the cylinders to be 
completely covered. is the piston fitted with five piston 
rings ; P the connecting rod working on the gudgeon pin 
Q fitted with a solid bush. R is the crankshaft, and it will 
be noticed that the cranks are at 180 degrees with each 



OIL AND PETROL ENGINES 



241 



other. The main bearings are shown at 8 and are of 
considerable length. The bottom ends T of the connecting 




Fig. 73. — General arrangement of Thornycroft G" x 8" Marine Petrol or 
Paraffin Engine — Side view. 

rods are adjustable, and it will be noticed that to assist 
lubrication the cap and bottom half brasses are left slightly 
narrower than the top half. The pinion ( T on the crankshaft 
drives two fibre wheels V connected to the half-speed 

R 



242 THE INTERNAL COMBUSTION ENGINE 



shafts. The free-wheel starting arrangement is shown at 
W, together with the handle and chain wheels. The 
sparking plugs are shown at XX and are of the positive 
make-and-break type worked by tappets YY. Advance 
sparking gear is worked by the lever Z, and half compression 
for starting by the lever shown. The exhaust collecting- 
branch is water-cooled. 

The makers claim that this engine is exceedingly simple 
to work even when very little practical knowledge is avail- 
able ; the motor canoes Spider and Sandfly supplied to 
Southern Nigeria have for some time been running under 
the care of native drivers, and they proceed long distances 
up the Cross River without white supervision. It is stated 
that the Spider had at the end of November, 1907, done 
10,000 miles, and the Sandfly somewhat less ; the Spider 
is a screw-in-tunnel boat, and the Sandfly a stern-wheel 
boat. 

112. The Petrol Engine.— The principle of working in a 




Fig. 74. — 16 H.P. Two-cylinder Albion Engine. 



OIL AND PETROL ENGINES 



243 



petrol engine is just the 
same as that of a gas or oil 
engine — so much so that 
petrol engines have not in- 
frequently been coupled up 
to suction producers and 
run as gas engines. Al- 
though this" is so it must be 
borne in mind that owing 
to differences in the nature 
of the working fluid the pro- 
portions of the engines re- 
quire to be designed separ- 
ately for each method of 
working. Thus the ports 
of a petrol engine will be 





Fia. 76. — Section through cylinder h;i\ ing 
inlet and exhaust valves on opposite sides 
— as for instance in Albion Engine. 



Fig. 75. — Typical piston of petrol 
engine showing covering of head and 
method of fastening the gudgeon pin 
upon which the connecting rod 
works. 



too small for efficient 
working as a gas 
engine. In a petrol 
engine the working 
fluid is a mixture of 
air with about 2 per 
cent., by weight, of 
petrol vapour. This 
mixture is formed 
by admitting both 
air and petrol to a 
device called a car- 
buret tor (about 
which more will be 
said presently). From 
the carburettor the 
mixture passes to the 
engine — most often 
through a throttle 
valve of the butter- 
fly wing variety. 
The proportions of 
air and petrol are 



244 THE INTERNAL COMBUSTION ENGINE 

adjusted by having variable inlets for the air and control- 
ling them by hand or by a governor. Illustrations are 
shown of petrol engines of the Albion, Lanchester and 
other types. Both are well known and deservedly popular. 
One, two, three, four, six or eight cylinders may be used 
to make up one engine. The cheaper cars usually have 



• 


&r Jwii^Ldbir i'hP Tr H Si 




-!.-:--T"Tr-ir** i i 








'J^i'l- XI 1SIIM 

r, T r - %.~r-~~~- "^-^z ... . urn* Ul ■' 




^asr 







Fig. 77. — 24 H.P. Four-cylinder Albion Engine. 

one or two, the four-cylinder car represents the medium, 
and six cylinders are most commonly fitted to the very 
best cars. Eight cylinders have only been tried experi- 
mentally so far. Marine engines may have any number 
of cylinders. The more cylinders an engine has the more 
uniform is the turning moment and the lower the speed at 
which the engine can be run without stopping. This is an 
important point, and it is usually discussed under the 
title of " flexibility. " A common speed for full load working 
is 1,000 revolutions per minute, and it is often convenient 
to be able to run at much lower speeds. Throttling 
the mixture has this effect and with a six-cylinder 
engine there should be no difficulty in getting down to 150 
revolutions per minute. If one attempted to do this with a 



OIL AND PETROL ENGINES 



245 



one or two-cylinder car the result would be to stop the engine. 
To get lower speeds, therefore, with small engines one has 




be 



to " change speed " as it is called. This brings us to the con- 
sideration of the mechanism by which the power of a petrol 
engine is transmitted to the road wheels of a car. The reader 



246 THE INTERNAL COMBUSTION ENGINE 



will best understand this by forming a picture in his mind 
(see Fig. 80) : — The engine 

is fitted to the car so that -2 

the crankshaft points in 
the direction of motion of 
the car ; this shaft is con- 
tinued from the front of 
the car to the back and its 
continuation is called the 
propeller shaft, owing to its 
being similarly placed to 
the propeller shaft of a 
screw steamship and 
actually being the propeller 
shaft when used in a 



thus 




Fig. 79. — Section through cylinder 
having both valves 
side. 



marine motor. This shaft transmits power by bevel or 



OIL AND PETROL ENGINES 



247 



chain gearing to the rear shaft of the car, which of course is 
at right angles to it. In order to be able to alter the velocity- 
ratio between the engine and rear shaft a gear box is fitted 
to the end of the propeller shaft which acts like the back- 
gear of a lathe, that is to say by sliding gear wheels in 
and out of action, by means of a lever worked by hand, 
the velocity-ratio can be altered at will. 

Readers who are familiar with motor cars and their 
engines will notice that the engine has been spoken of as 
being on the front of the car. This is by far the most com- 
mon practice, although cars are built, such as the 10 h.p. 
Adams, in which the engine is placed under the back part 
of the body of the car. It is simpler, however, to the novice 
to picture the most common form of disposition of parts 
and to inquire into the others afterwards. 



FlywheeJ^ 



Clutch Spring 




Crankshaft 



Clutch Bolt 
of Crankshaft 



Sleeve carrying Male Clutch Member 
Clutch Collar 



Fig. 81.— Friction Clutch. 
.Albion Typo. 



113. Reverting to the typical car we have been thinking 
about, with the engine well forward under its " bonnet " — 
it is now desired, let us say, to start it. To do this it is 
necessary to give it a few turns by hand and for there to be 



248 THE INTERNAL COMBUSTION ENGINE 

no load in the engine whilst doing this. To remove all load 
it is necessary to disconnect the engine temporarily from 
its propeller shaft and from the gear-box, differential, etc., 
which the propeller shaft drives. To do this a cone clutch 
is introduced between the engine shaft and the propeller- 
shaft. Fig. 81 is an illustration of such a clutch. The clutch 
consists of two conical parte, the inner one having a leather 
facing, which are pressed together by a spring and separated 
by depressing the clutch pedal. When the two parts are in 
frictional contact the engine drives right through, but when 
the clutch pedal is pressed down, only the left half rotates. 
It is in this latter position when the engine is started. To 
get the car into motion the foot is gently raised, and the two 
parts of the clutch come into contact and rotate as one piece. 
There are other varieties of clutch, but this is the commonest. 
The clutch is often made a good deal heavier than would 
otherwise be necessary, in order to provide a " flywheel 
effect," which is of great use to the engine, especially when 
there are only one or two cylinders and there are there- 
fore several strokes in which no explosion occurs. The 
gear-box takes the most different forms according to 
the type of engine selected. From the gear-box a short 
length of shaft runs to the bevel gear in the " differential " 
as shown in Fig. 80. This gearing is similar to that used 
on old-fashioned tricycles and enables the two road wheels 
to adjust their relative speed when turning corners. The 
actual transmission to the road wheels is either by — 

(a) Chain drive, 

(6) Live axle drive, or 

(c) Pinion drive. 

In the first named alternative, chains are used much as 
in a bicycle. The disadvantages are noise and stretching 
of the chains under the sudden and heavy loading applied. 
The live axle drive is now becoming the more popular and 
is about 3 per cent, more efficient than the chain in respect 
of power transmission. The pinion drive is used for heavy 
vehicles for which the other two types would be unsuitable. 

The mechanical efficiency of the transmission from engine 
cylinder to road wheels is variously stated as anything 



OIL AND PETROL ENGINES 



249 



from 60 to 80 per cent. The lower figure is the nearer one, 
and probably 65 per cent, is as much as can be expected. 
The following table shows generally the way in which the 
losses are incurred. 

Power available at road wheels 

Lost in gear-box . 

Lost in engine friction 



65 per cent. 

15 

10 

7 

1 

2 
100 



Lost in differential 

Lost in clutch 

Lost in drive 

I.H.P. developed in cylinder. 

The author has travelled on cars driven by one, two, four, 
six and eight cylinders at speeds varying between one and 
sixty miles per hour, and on cars fitted with each of the 
alternative devices above mentioned. It is wonderful that 
such generally similar and generally satisfactory results 
are achieved in each case, but the difficulties which 
have yet to be surmounted are the ensuring of long life of 
the engine and other parts of the car, the attainment of 
high thermal economy of the engine and, what goes with it, 
the complete burning of the petrol used so that no carbon 
monoxide or hydrogen can be found in the exhaust. 

The above description and illustrations of important 
parts should render the reader capable of following the more 
detailed treatment of the vital part of the mechanism which 
now follows.* 

114. Carburettors. — The function of a carburettor is to 
intermingle the petrol or other fuel with the air so that an 
explosive mixture is formed which can be admitted forth- 
with to the cylinder. It is possible to allow only a portion 
of the air to pass through the carburettor and then to add 
additional air to the mixture so as to bring it to the required 
proportional composition. Or, on the other hand, the whole 
of the air may be passed through the carburettor. Both 
methods are commonly in use, often combined on one engine 
as will presently be seen. When petrol is used the " inter- 

* Readers desiring further information as to the construction 
and working of motor cars should read Mr. Strickland's " Manual 
of Petrol Motors and Motor Cars. 



250 THE INTERNAL COMBUSTION ENGINE 




Fig. H2.—A. 
Controller. 
Engine. C. 



Petrol inlet 
B. Inlet pipe 
Needle valve 



regular flow of petrol. 



mingling " is largely between 
the vapour of petrol and air. 
Some petrol however comes 
over in the form of a liquid 
spray, and air carrying such a 
spray (or even coal-dust for 
example) is quite easily explo- 
sive. 

The " intermingling " is 
caused in one of two ways : (1) 
by the jet method, or (2) by 
the surface evaporation method. 
In the former the jet may be of 
either of the varieties shown in 
Figs. 82 and 83. In the former 
the air sucked through B, on the 
opening of the valve, causes petrol to rush up the pipe A, past 
the screw-adjusted inlet opening to a small hole on the 
conical seating of the valve. The lift of the valve there- 
fore not only admits air but uncovers the small petrol hole 
up which a jet of petrol at once squirts. Then the petrol, 
having so low a vaporizing point, at once turns into vapour 
and forms with the air an explosive mixture. The screw ad- 
justment — or needle 
valve as it is called — 
allows the richness of 
the mixture to be ad- 
justed. Fig. 8 3 shows 
a better and more 
f amiliar way of doing 
the same thing. Air 
is sucked in past the 
jet F and out by the 
opening K. In rush- 
ing past the jet it 

SUcks Up a petrol Fia g3 _ Jet Carburettor and Float Chamber. 
Spray which evapor- A. Petrol inlet. B. needle valve. C. Float 
a tpcs sf<s it o-ptq TYiiYPfl which closes the needle valve B through the levers 

ates as ir gets mixea D when the petrol reaches the level EE F p etro i 

with the air. On the jet. G. Air nozzle. K. Inlet pipe to engine. 




OIL AND PETROL ENGINES 



251 



left of the figure is seen what is called a float chamber 
for keeping the petrol level constant. It operates much 
as does a ball and cock feed to a water cistern. B is a 
needle valve which gets pushed down on to its seating 
by the levers D when the float C rises to the top. This 
stops more petrol coming in until the petrol-level sinks so 
much as to let the float down till the levers open the needle 
valve again when more petrol flows in. The weight of the 
float is so adjusted that the petrol-level is kept at just the 
right height. It is the custom to have the petrol standing 
just below the top of the jet, but it works even if standing 
much below the top of the jet. Evidence of this has been 




-Aid 



FLOAT FEED. 



PETROL 



"PRESSURE INLET 



Fig. 84. — Lanchester Surface Carburettor. 

afforded to the author by noticing instances in which the float 
chamber has been sucked quite dry during the running of 
the engine when the pipe A has got choked up in some way. 
The principle of the working of the jet will be gone into later. 
An efficient type of a surface carburettor is shown in Fig. 84, 
which illustrates the carburettor used on the Lanchester 
engine. The principle of its working is obvious from the 



252 THE INTERNAL COMBUSTION ENGINE 



diagram. The air passing over a large petrol-soaked surface 
takes up petrol vapour. All the carburettors described work 
best with a certain velocity of flow of the air. When, how- 
ever, the engine runs very fast or very slow the air velocity 
changes accordingly so that the carburettor sometimes gets 
more air, and sometimes less, than it wants. If the flow of 




Fig. 85. — Krebs Carburettor, in which the opening to the extra air supply 
is controlled by the suction of the engine. It works excellently. 

air be increased it is found that too much petrol is taken 
up, so it is customary to arrange for only part of the air 
to pass the jet (say) and for the rest to be added to the 
mixture without passing the jet at all. It is best to arrange 
for this adjustment to be made automatically and Figs. 85 



OIL AND PETROL ENGINES 



253 



and 86 show how this can be done. The former shows the 
Krebs automatic carburettor. When, owing to increase of 
piston speed, the suction on the air increases, the leather 
diaphragm is sucked down against the weak spring P 
and opens a valve at M so that air can flow in and mingle 
with the air which has entered at G and has passed the 
jet F. L is the throttle valve controlling the quantity of 
the mixture which is allowed to pass to the cylinder by the 
pipe K. Fig. 86 shows another way of doing the same 
thing. As the suction increases the extra air comes in 
through the valve M and joins at K the part which has come 




Fig. 86. — Automatic Carburettor working in a generally similar way to 
the Krebs. The opening of the valve M depends on the suction. 

in over the jet F. There are many other ways, easily 
devised, of applying the same principle. 

Owing to the heat absorbed by the evaporation of the 
petrol it is usual to warm the entering air slightly. This 
is done by putting the air inlet pipe nozzle close up to one 
of the exhaust pipes so that the air in rushing past the hot 
pipe gets warmed slightly. Of course the fact that the whole 
of the carburettor is under the warm engine bonnet helps 
to keep the temperature from falling too low. 

When paraffin is used as a fuel much more heat is neces- 



254 THE INTERNAL COMBUSTION ENGINE 

sary, as paraffin does not vaporize nearly so easily as petrol. 
The general form of float and jet is, however, the same. 

115. The carburettor is usually controlled by the governor 
actuating a throttle, although sometimes it is the lift of the 
exhaust valves that is regulated. The former is the more 
common method, and as an instance of it the following 
description of the Murray governor is given. It is an 
instance of a governor which does as much as any governor 




Floating Lever 



Advance ignition 
Connecting Rod 



Speed Air Port 
Connecting Rod 



Fixed Fulcrum Pin 



iPif •• •• ••Mm 

!•••••• Mr 



Fig. 87. — Arrangement of Murray Governor and Automatic Carburettor. 

fitted to any car and far more than most. It controls the 
throttle, the ignition and the extra air port. By leaving 
out one or other of these any other type of governor is 
sufficiently well represented. Mr. Murray thus describes 
it : " The rotary portion of the governor is of the usual 
centrifugal type, but instead of being designed to act at 
one given speed the sleeve actuated by the centrifugal pull 
of the balls against the spring begins to move off its lower 
stop at an engine speed of 180 revolutions per minute and 



OIL AND PETROL ENGINES 



255 



does not reach the top limit of its travel until the engine 
attains a speed of 950 revolutions per minute. The travel 
of the collar along the shaft is much greater than is usual in 
centrifugal governors. The connecting link from the 
governor sleeve to the throttle valve is of a length variable 
by the control lever. In other words, the relative position 
of the throttle valve to the governor sleeve can be varied 
by the driver. This control lever therefore fixes the speed 
or point of travel of the governor sleeve at which cut-off 
is to take place, and thus sets the engine instantly to run 



12 



oovernor 
Characteristic 




«o 


o 


t. 


* 


Q 






04 "J 


i 




o 

to 


CO 


V 


C 


o 


fa 



200 QQO 600 800 1000 R.P.n. 

Fig. 88. — Characteristic Curves for Murray Governor. 

at any desired speed within the given range. The throttle 
valve is of the piston type, so that it can overshoot its cut-off 
position in either direction by an amount equal to the total 
travel that the governor sleeve can give it. In its lowest 
position the control lever sets the throttle valve just open 
when the motor is at rest, therefore the moment the motor 
is started up and reaches a speed of 180 revolutions per 
minute the throttle valve commences to close and the engine 
will run about 200 revolutions per minute light. If set about 
one-third up, the engine will attain a speed of about 600 
revolutions per minute, when the governor will once more 
close the throttle valve, for, as will be scon by a reference 
to the governor characteristic in Fig. 88 ( his speed corresponds 



256 THE INTERNAL COMBUSTION ENGINE 

to about one-third of the travel of the governor sleeve. If 
the control lever is set right up to its control limit, the engine 
will rise to 950 revolutions per minute before the governor 
closes the throttle. About one-eighth of the total travel 
of the governor sleeve is all that is required to give practically 
full admission, so that when set at any given speed for light 
load a comparatively small drop in speed will suffice to 
open the throttle full up. Obviously, therefore, at whatever 
speed the driver sets the control lever, the governor will 
hold the car at practically a constant speed on the level, 
uphill and downhill, without any intervention on the part of 
the driver ; so long, of course, as the gradient is not too 
steep for the engine to tackle, on the gear on which it is 
running, or the down grade so steep that it is sufficient to 
drive the engine above its normal speed even with all 
petrol cut off. The throttle valve is attached to the centre 
of a double lever, one end of which is controlled by the 
governor, the other by the control lever under the direction 
of the driver. Setting the control lever at the lowest point, 
the governor being at rest, the throttle is just open, conse- 
quently a very small speed of the engine is sufficient to close 
the throttle throughout the range of the control lever. At 
whatever speed it is set the engine will follow up to this 
speed, and the governor will hold it there." 

The author has had experience of the manner of 
working of this governor both by actual driving and 
by watching its action, and he has formed a high opinion 
of it. It performs its manifold functions better than the 
average man, but not, of course, so well as the specially 
skilled driver. As will be understood on reference to the 
diagrams shown — the governor is aimed to deal with the 
average case. 

The type of carburettor generally used with this governor is 
shown in larger size in Fig. 89. In the annular chamber round 
the vena contracta (as it is called), at the right side of the illus- 
tration, there is an air port — called the speed air inlet — 
covered by a sliding shutter (see dotted lines) which is con- 
nected to the mechanism of the governor as already illustrated. 
The shape of the port is so designed as to give the area of 



OIL AND PETROL ENGINES 



257 



opening shown to be necessary by the curves experimentally 
obtained (see Fig. 88). Another port — called the load air 
inlet — is opened proportionately to the opening of the 
throttle valve so that the mixture may be kept constant as 



Induction Pipe 



Governor Vdlve 



/»' .' fT -"\ Speed Air Inlet 



Governor Valve Spindle 




Hoi Water Jacket 

Load Air Inlet J \,'' ) 

Lever for operating Speed Air Inlet 

Fig. 89. — Albion Carburettor. 



far as possible and free from any interference on the part 
of the driver. 

116. A carburettor which has no additional air inlet and 
relies upon keeping the correct mixture by manipulating the 
jet is the White & Poppe, which is fitted on the Maudslay 
and other engines. The principle is to keep the flow of air 
past the jet constant as regards its velocity, and to arrange 
for the throttle opening to be proportional to the size of 
the opening through which the petrol passes up the jet. 
This is done by placing the jet in the centre of a chamber of 
circular cross-section across which the stream of air passes. 
This chamber is encased in a metal sleeve and the whole 
has a circular air- way drilled through both sides. Then as 
the jet chamber is caused to rotate slightly the air passage 
through is restricted and this restriction is made to affect 
the jet also owing to the hole of the jet being drilled a little 
eccentrically and a cap fitting on to the jet being similarly 
drilled and so fixed that as the jet and chamber rotate 
through an angle the effective jet aperture is decreased at 
the same time as the area through the air passage. That 
these two openings increase and decrease in the same ratio 
is ensured by the fact that in each case it is a circle 

s 



258 THE INTERNAL COMBUSTION ENGINE 

sliding over a circle and that both are fully opened and 
fully closed together. It is stated that with this carburettor 
as much as nine to ten miles per gallon have been accom- 
plished on a car which otherwise would probably have only 
run seven miles. In the 1908 Commercial Vehicle Trials of 
the Royal Automobile Club a Maudslay vehicle fitted with 
this carburettor ran no less that 9-05 miles per gallon, a per- 
formance greatly in advance of that of any other vehicle.* 











mi ■■■■^misL^mr *m 










Jhh^B H 





Fig. 90. — Cottrell Carburettor as applied to Two-cylinder Engine. 

117. The Cottrell Paraffin Carburettor.— This carburettor 
is illustrated in Figs. 90 and 91. It is also shown diagram- 

* See also Chapter IX. 



OIL AND PETROL ENGINES 



259 



matically in Fig. 92. In this diagram G is the pipe which 
receives the air and paraffin spray coming over from the 
jet in the carburettor marked H in Fig. 92. When the 




Fig. 91. — Side view, of .Cottrell .Carburettor. 



fuel gets to the branch pipe it divides right and left to 
either end of the vaporizer, M. M is shown in section 
at the lower right-hand side of the figure. It consists of a 
corrugated pipe which is surrounded by hot exhaust gases 
and conveys in its interior the air and paraffin mixture. 
This corrugated pipe has to be kept hot. Paraffin will not 
work the engine until it is hot. In starting from the cold, 
provision is therefore made for working on petrol for two or 
three minutes and then, the pipes M having got hot, the 
paraffin is turned on. At B there passes a mixture of heated 
air and paraffin vapour. The air is much less in proportion 
than would work the engine, so at F an adjustable inlel 
is fixed to admit more air until the mixture is of the 
usual proportions. The idea of not letting all this air 
in before the vaporizer is reached is that with a less propor- 



260 THE INTERNAL COMBUSTION ENGINE 

tion of air the paraffin particles get more effectively heated 
and the velocity of passage through the vaporizing tube 
is slower. The whole process is therefore quite simple. 

To recapitulate — a little air with a heavy proportion of 
paraffin spray is passed through a very hot corrugated tube 




Fig. 92.— Cottrell Paraffin Carburettor. Shown diagrammatically. 



and vaporized. Then it is diluted with a lot of cold air to 
the correct proportion and passed into the inlet port of the 
cylinder. These carburettors have been known to work 
successfully on motor cars for many thousand miles. A 
variation of it has been tried whereby air only was passed 



OIL AND PETROL ENGINES 261 

through the star tubes M, then through a lagged pipe 
to the other side of the cylinder and thence through 
an ordinary carburettor such as is designed for use with 
petrol. This alternative arrangement also worked well, 
but hardly seems so effective as the unmodified type of 
Cottrell carburettor. To begin with, the paraffin draws all 
its heat from the air which sweeps it along, instead of by 
actual rushing contact with the hot vaporizer tubes. 
Further, the mixture enters the cylinder much sooner 
after its creation than in the other arrangement, and it 
is evidently advantageous to allow time for the paraffin 
vapour and air to mix intimately with each other. The 
patentees state that the compression pressure for working 
on paraffin should not exceed 65 to 70 lb.* per square inch 
and that magneto ignition is preferable to coil and battery 
ignition. They admit also that a drop in horse-power in 
the engine must be expected of 10 to 15 per cent, com- 
pared with the horse-power obtainable when using petrol. 
This loss is chiefly due to the higher temperature of the 
entering fuel which with a given cylinder volume naturally 
reduces the weight of charge admitted. There is also a 
loss owing to the necessary lowering of the compression 
ratio and consequent lowering of thermal efficiency. This 
lowering of the compression pressure is due to fear of back- 
firing (through the inlet ports before they have quite closed) 
caused by the charge being pre-ignited owing to the tem- 
perature rising to the point at which paraffin vapour and air 
ignite spontaneously. Lowering the compression pressure 
naturally lowers the compression temperature. 

There is a gain, however, in that the calorific value 
of paraffin per pound is about 18,000,000 ft. -lb. against 
about 15,000,000 for petrol ; also an advantage is found in 
the smaller consumption of lubricating oil owing to the 
lubricating properties of the paraffin itself. 

As petrol costs two or three times as much to buy as 

paraffin it is clear that even when allowance is made for a 

drop in horse-power there is still a considerable gain in 

economy from the financial point of view. The patentees 

*Many engines work at 80 lb. per sq. inch compression. 



262 THE INTERNAL COMBUSTION ENGINE 



of the Cottrell carburettor have published the following 
comparison of costs of the two methods of working — 



Pleasure Vehicles. 
Horse-power. 


Price 
Petrol and 

Paraffin 
(per gallon). 


Cost 
per 
Mile. 


Miles 
aver- 
age 
per 
gal. 


As- 
sumed 
Mileage 

per 
Annum. 


Mileage 
to be run 
to Cover 
Cost of 
Carbu- 
rettor. 


6 h.j 


). single cylinder . 


1/2& U. 


f 
I 

f 


•4 Y 
•17 J 
•46 ) 


35 


8,000 


4,460 


10 „ 


double ,, 


1 12 & 6d. 


1 
f 


•20 j" 
•56 { 
•24 j 


30 


8,000 


6,500 


15 „ 


four ,, 


1 /2 & 6d. 


\ 


25 


9,000 


7,000 


30 „ 


»» ?? 


1/2 &6d. 


\ 


•7 I 
•3 ) 


20 


10,000 


6,500 


50 „ 


55 55 • 


1 /2 & 6d. 


{ 

f 


•93 ) 

•4 | 

1-75 \ 

•75 J 


15 


10,000 


4,800 


100 „ 


55 55 


1 /2 & 6d. 


1 


8 

1 


10,000 


3,300 


Commercial Vehicles. 








1 ■ 






10 h.p. 


single cylinder 


1 /- & 4|d. 


J 

t 


•48 | 
•18 ) 


1 25 

1 


50,000 


3,500 


20 „ 


double ,, 


1 /- & 4|d. 


f 
1 


•6 \ 

•225) 


20 


50,000 


4,200 


20 „ 


four ,, 


1 /- & 4|d. 


f 
1 


■8 I 
•3 ) 


15 


45,000 


4,550 


50 „ 


55 55 


l/-&4id. 


I 


1-2 ) 

•45 J 


1 
10 


45,000 


3,600 



Figures such as the above depend very largely upon the fuel 
consumption per mile run by the car. From tests carried 
out by the author it appears that a 20 h.p. transport car 
weighing about 2 tons and carrying 2 tons, will run eleven 
miles on one gallon of petrol, and ten miles on one gallon of 
paraffin when using the Cottrell carburettor as illustrated. 

118. Milnes-Daimler Carburettor. — The Milnes-Daimler 
firm have recently introduced a new form of carburettor 
which is claimed to be equally suitable for petrol, alcohol or 
paraffin. It has a jet and float-feed of the usual type. 
The air passes the jet in an upward vertical direction, not a 
horizontal one. The working of the instrument is best 
described by stating what happens to the air from the mo- 
ment of admission until it is delivered to the cylinder. Air 



OIL AND PETROL ENGINES 263 

is drawn first of all into a jacket surrounding the exhaust ; 
this raises the temperature to a considerable degree, and on 
leaving the jacket the air pipe has holes in it controlled 
by a rotating sleeve of the usual form in order by admitting 
more air to reduce the temperature to the requisite extent. 
If petrol is to be used a good deal of cold air is admitted ; 
if paraffin very little, if any, additional air is required. The 
air then passes upwards around the jet. The jet itself has a 
hollow truncated cone above it (the angle of the sides being 
about 15°) so that if the cone is lowered the air passage is 
reduced in area. In this way the top of the jet is in the 
middle of an annular space through which air flows and the 




Fig. 93. — Positions (a) and (6) of Hood in Milnes-Daimler Carburettor. 

area of this annular space can be varied in a ratio of about 
1 to 2. The idea is to increase or decrease this area so 
that the velocity of the air past the jet may be kept from 
varying. 

The " truncated cone " sleeve is raised and lowered by 
the governor and it is made in one piece at the top with a 
thin brass sliding cylinder having ports in it. These ports 
are opened as the sleeve rises and extra air (cold) is taken 
in. The whole gaseous mixture then passes out through 
upper ports in the same cylindrical shell, which being con- 
trolled by the governor causes the necessary throttling. 
The governor, therefore, not only works the throttle but 
also the extra air inlets and the cone sleeve just above the 
jet. Adjustment for different densities and qualities of 
petrol can be made by slightly rotating the cylindrical 
shell and then fixing the position by screws. An engine 



264 THE INTERNAL COMBUSTION ENGINE 

fitted with this type of carburettor is tested on the bench 
and set so as to give greatest petrol, or other fuel, economy 
at the normal speed, say 800 revolutions per minute. If 
the governor really could by its adjustments of the conical 
sleeve keep the air velocity constant the rate of flow of 
petrol would be constant, and the richness of the air 
mixture immediately after passing the jet would be in- 
versely proportional to the effective area of the air passage 
in the annular space around the jet. The extra air is 
admitted to keep the quality of the mixture the same. 
All this depends on the action of the governor, that is to 
say on the engine speed, and its successful working depends 
on a proper shape for, and adjustment of, the various 
openings and ports. 

119. Another type of paraffin carburettor is the "Broom 
& Wade." Here the paraffin is supplied to a valve type 
of inlet — as shown in Fig. 82, and enough air is admitted 
to get the requisite quantity of fuel to enter and scrub its 
way past several layers of wire mesh which serve to break 
up the liquid. On the suction stroke the valve opens to 
admit this to the cylinder, and later on in the stroke a 
valve opens which admits through an inlet in the cylinder 
wall the requisite extra air to make the mixture correct. 
The firing point (low tension magneto) is well back from 
the cylinder, at the end of a sort of combustion chamber 
which is made in one with the carburettor (an arrangement 
which makes a separate jacketing of the carburettor by 
exhaust gases unnecessary). The engine is started up on 
petrol, then changed over to paraffin, and the control is 
through the medium of a lever which affects the lift of the 
suction valve. At heavy loads there is likelihood of 
pre-ignitions owing to the high temperatures reached, and 
it is therefore arranged that water can be admitted with 
the extra air, the water being carried in past the valve in 
much the same way as the paraffin is carried past the 
suction valve. 

120. The Thornycroft type of paraffin carburettor is 
illustrated in Fig. 94. Its manner of working is generally 
similar to that of the Cottrell, but the heating surface is 



OIL AND PETROL ENGINES 



26i 



less in proportion. 
It works well in 
practice and its 
mode of operation 
is as follows : — 

The oil is drawn 
into the vaporizer 
together with a cer- 
tain amount of air 
by the suction of 
the engine ; this 
mixture is then 
passed through a 
tube which is kept 
heated to a fairly 
high temperature 
by the exhaust 
gases coming 
from the engine. 
This thoro u g h 1 y 
vaporizes and in- 
timately mixes the 
vaporized oil and 
air. The mixture 
is then passed 
through a spiral 
separator, which 
separates any solid 
matter from the 
vapour, is mixed 
with extra air as 
required to form 
an explosive mix- 
ture, and then 
passed through the 
throttle to the 
cylinders. In the 
drawing, A is the 
inlet valve for both 




Thornycroft Paraffin Carburettor, 



oil and air, the valve being under 



266 THE INTERNAL COMBUSTION ENGINE 

the action of spring B, which normally keeps the valve 
closed and the oil supply C shut off ; the oil enters 
by holes in the seating. The mixture then passes along 
the annular space EE which is kept heated to a high 
temperature by the exhaust flowing through the centre 
of this annular chamber as shown at F. The annular 
chamber it will be noticed is fitted with gills G to enable a 
maximum amount of heat to be supplied to the mixture. 
H illustrates the separator for removing the solid particles 
from the mixture, and J the " extra-air " inlet, under the 
control of the spring K. The tension of this spring and 
also of that governing the inlet of the mixture to the 
vaporizer can be varied by a screw and nut as shown. This 
adjustment is made when the engine is on the test-bed 
before the brake trials. The outlet from the vaporizer to 
the motor is by pipe L. 

121. Theory of Jet Carburretors. — The energy equation for the 
flow of any fluid (liquid or gas) is as follows — see Perry's Applied 
Mechanics, p. 533 — 

v 2 [dp 

-tz — + I — + h = constant ( 1 ) 

2# J w 

This is true for any stream line. 
v = velocity of fluid. 
#=32-2. 
p = pressure. 

w = weight of unit volume or density. 
h = potential energy due to height. 
If this equation be applied to the flow of air through the carburettor 
due to the suck of the engine, it may be simplified in many ways. 
We want to find the amount by which the pressure in the rushing 
air is made lower than the atmospheric pressure owing to the suck of 
the engine pistons. When the air is at rest equation (1) becomes 
f dp 

+ /inconstant (2) 



/ w 
when air is rushing with velocity v then 



l dp 



v 2 
~-x — + I - -" + h = constant (3) 

Now h and h are the same, as the air may be taken to flow 

horizontally. 

Therefore equating (2) and (3) we have 



w ~ 2g + ) w w 

but if unit weight of air be considered, w= y, where V is volume, 



J 



OIL AND PETROL ENGINES 267 

and p Vy = constant is the approximate law connecting p and F when 
no heat is given or received. y= 1-41 as usual. Therefore the expres- 
sion becomes 

w 



i_ r> dp 

J py 

J, ] 
= yc y t 



V.dp 
dp 



1 1--1 

7 



7-1 

Substitute in equation (4) and then we have 

i r 7- 1 r- * 



Let difference in p and p be called 8p, then since 8p is small 

= -^-jcTj) y {( 1+ ~^) T — * } a PP roximatel y- 

which reduces to -£■ = —?— (5) 

2g w 

From this we have 

S P = ^ (6) 

The pressure at the top of the petrol jet is therefore lower than 
the pressure on the surface of petrol in the float chamber by dp 
where ftp is given by equation (6). A somewhat similar line of 
argument to the above will show that the flow of the petrol is 
determined by a formula of [the same type. For a liquid such as 
petrol in which w is independent of p, equation (3) becomes 

v 2 p 
~<r + — + 7i = constant (7) 

2g w 

Applying this equation to the stream lino which begins at the 
free petrol surface in the float chamber and ends in the jet spray 
and assuming that the top of the metal jet is a height h above 
the petrol level (so that petrol has to rise through a height h in the 
jet before it gets to the actual nozzle) we have 
hp v 2 

w 2g 

where v— velocity of flow of petrol and w=density of petrol (i.e. 
the weight per cubic ft.). 



268 THE INTERNAL COMBUSTION ENGINE 

Therefore -|^= ~—h 

2g w 

Now by equation (6) : — 8p= ~^- 

V 2 WfiVr) 2 , 

so that -s- = °° — ^ 

2gr w2# 

or v 2 = —v 2 —2gh (8 

This shows that v , the velocity of air flow, must have the value 

w 

2gh — before any petrol will flow at all. It is a matter of common 

experience that if the rate of air flow doubles the petrol flow more 
than doubles. Let us see if this is so according to this formula. 

First, let v = a, and then equal 2a. The ratio of the square of 
the petrol flow in the second case to that before should therefore, 
according to experience, be more than 4 — by formula (8) it is 

^-4a*—2gh 



V 



^a*-2gh 

w y 



6gh 
= 4 + (9) 

a 2 — 2gh 

which it will be seen does exceed 4, as experience records. 

It will be interesting to get some quantitative figures for this. 
What, for instance, will be the ratio if h =004 feet (or \ inch) and 
a is 5,000 ft. per minute ? 

Petrol has a density of about 0-72 so that 1 cu. ft will weigh 
0-72x62-3 = 45 lb. Whereas 1 cu. ft. of air weighs 0-085 lb. at 
atmospheric temperature and pressure. 

According, therefore, to equation (9) the square of the ratio of 

the two petrol velocities will be 

6x32-2x00 4 

4 + 0-085 

—££■ a 2 — (64-4X0-04) 

•7-72 
= 4 



a' 
530 _ 2 ' 57 



The critical velocity is clearly 

v = \/2gh~= /\/2x 32-2X0-04X 530= ./ 1370= 37-Oft./sec. 

= 2,220 ft. per min. Until, therefore, the air had this velocity 

5000 
no petrol would be carried along. Since a= ~ fin the equation (9) 

becomes 



OIL AND PETROL ENGINES 269 

7-72 7-72 

4 + 13-0— 2-57" + 10-4 

= 4-74. 
So that when air velocity increases from 5,000 to 10,000 ft. /min. 
the petrol velocity is 2-18 times as much and the mixture therefore 
1 -09 times as rich or 9 per cent, richer. This means that the amount 
of petrol present per cubic foot of air increases by one-eleventh part. 
This inequality is really most marked when the velocity of the air 
is only a little more than what is necessary to feed the petrol, thus 
if the air velocity be increased from 2,500 ft. /min. to 5,000 ft. /min. 
the petrol sucked along would be increased by no less than 290 per 
cent., i.e. the quantity of petrol would be 3-9 times as great, giving 
a richness of mixture 1-95 times or nearly double what it was before. 
In this case, therefore, about twice the quantity of air would be 
needed, i.e. as much again must pass the additional air inlet as 
already passes the jet. This theory accounts for part of the extra 
air needed. It does not, however, take account of the effect of 
eddies, that may circulate around the jet at high air velocities, and 
so complicate the problem a great deal. 

122. It is possible that there are still further reasons 
why extra air is needed when the engine speed increases, 
but the above are certainly some of them. The air velocity 
is highest when the engine is running fast and the throttle 
wide open. When the engine is running fast but is much 
throttled, as in a car running very fast down a slope, 
the vacuum behind the piston never gets filled up with air 
and the velocity of air past the jet is not therefore very great. 
Going up hill, however, on low gear the engine speed is 
high and the throttle wide open ; so that the velocity of air 
past the jet is not solely dependent on the engine speed. 
This makes control of the additional air inlet by the centri- 
fugally operated governor not as uniformly good as could 
be wished — it only approximates to extreme cases, fitting 
accurately the average only. When, however, as in the 
Krebs carburettor, for instance, the opening of the extra 
air inlet is controlled by the suction a much more constant 
mixture is obtained. A carburettor is usually set so that the 
right mixture comes away from the jet at low speeds with the 
extra air inlet closed. Then as the speed rises additional air is 
allowed to pass in. From the preceding calculations it will 
be clear that one cause of the lack of proportionality between 
the air and petrol velocities is the fact that the petrol 
cannot be allowed to stand at a level equal to the height 



270 THE INTERNAL COMBUSTION ENGINE 

of the jet, as if this condition were arrived at too closely 
there would be a risk of the petrol overflowing and 
running away to waste. A further reason which makes 
it necessary not to run the adjustment too finely is that the 
level of petrol in the float chamber is bound to vary some- 
what not only with the inclination of the float chamber 
but a ] so with temperature and quality of supply. In 
order to keep on the safe side the petrol level must be 
a good many millimetres below the top of the jet. 

123. Ignition. — Types of ignition are many, and the 
author proposes to describe several examples of modern 
methods, but before doing so it is necessary to say something 
about the principles upon which they work. The oldest 
form of all was to ignite the explosive mixture by a naked 
flame, which was put into communication with the cylinder 
through the medium of a sort of slide valve. It is quite 
obsolete now, but those interested in the history of the 
subject will find full details in Mr. Dugald Clerk's book. 

A later and much more successful form was Tube ignition, 
which consisted in having a short vertical tube, in com- 
munication with the cylinder end, heated externally by 
means of a lamp. After it had been running a short 
while the lamp could be removed (or the gas jet turned 
out) and the heat of ignition was enough to keep the tem- 
perature up to the requisite point. It is illustrated in Fig. 
95. The explosive charge is compressed by the inward 
movement of the piston, and a part of it passes into the 
ignition tube, the temperature of which raises the tempera- 
ture of the gases at the end of the stroke to just the 
correct temperature for explosion. It may seem as though 
it would be difficult to effect such a nice adjustment as 
this, but it works more successfully than might be expected. 
It appears as though the criterion that settled the moment 
of explosion, provided the circumstances were favourable, 
was the movement of the gases to expand just at the instant 
when the crank passes the dead centre and the piston begins 
its outward journey. This type of ignition has been very 
largely used for gas engines, but it is much less common 
now. It was also the first method employed on motor cars, 



OIL AND PETROL ENGINES 



271 



for which use it was obviously unsuitable and speedily 
fell into disuse. 

Another method of ignition often used with oil engines 
is to feed the fuel right into a hot combustion chamber 
connected to the cylinder. This method as carried out on 
the Hornsby engine has already been illustrated. It works 
very well and even crude Borneo oil can be vaporized and 
ignited in this way. The method employed on the Diesel 
engine is of the same general type. 

124. The chief method of ignition is the electric, and 
it bids fair to supersede all the others — indeed it has 



C.I. Shield __ 



Timing Valve 




Ignition tube (Clay) 
Asbestos Millboard Liner 

Gas Burner 
To Cylinder 



Fig. 95. — Ignition Tube and Timing Valve. 

nearly done so already. It is equally suitable for gas, 
oil or petrol engines. 

Electric ignition can be carried out by either (1) high- 
tension currents or (2) low-tension currents. In the former 
the voltage runs into many thousands of volts, and in the 
latter probably only into hundreds. The word "probably " 
is used, as unless an oscillograph is employed, it is very 
difficult to say exactly what the voltage is. 

The high-tension currents may be obtained in three ways : 
either (1) by batteries or cells furnishing current to an 
induction coil, or (2) by a small magneto-electric machine 
(called "magneto" for short) furnishing currents to an 
induction coil, or (3) by a magneto furnishing high-tension 



272 THE INTERNAL COMBUSTION ENGINE 



currents direct to the sparking plug (the name given to the 
spark gap in the cylinder). The low- tension currents are 
produced from a low- tension magneto. There are therefore 
many varieties of electric ignition and they may conveni- 
ently be set out thus — 

Electric Ignition 



I 

High tension 



. I 

Batteries or cells 
with induction 
coil 

I 
_l 



Magneto with 
induction coil 

(3) 



High-tension 
magneto 

(4) 



I 

Low tension 

I 

I 

Low-tension 

magneto 

(5) 



I I 

Coils fitted with Coils fitted with 

tremblers fixed " make-and-break " 
(1) (2) 

This makes quite a family tree and it is clearly a case 
in which an engine builder has a considerable variety of 
choice. There are five main varieties. The oldest is (1) 
and it is still seen fitted to some petrol engines, although 
(2) is more common ; (3) is relatively rare but is seen 
in the Eisemann system ; (5) is common practice and a 
most satisfactory method which is coming more and more 
into use. It is particularly suitable for engines which have 
to work in a tropical or sub-tropical climate. Method (4), 
however, is growing in popularity and it has the advantage 
of being simpler to apply to the engine than (5). 

Before describing methods (1), (2) or (3) it will be necessary 
to say something about the induction coil. To those who 
are versed in electrical matters it is enough to describe it 
as a transformer having a straight iron core and a high 
ratio of transformation. To others not so familiar with 
electrical matters a little more explanation will be necessary. 

125. Induction Coil. — The induction coil used on a car is 
similar to the usual type. It consists of a soft iron core 
generally consisting of a bundle of straight iron wires and on 
it is wrapped a layer or two of thick insulated copper conduct- 
ing wire of the primary — or low-tension — circuit. Over this 



OIL AND PETROL ENGINES 273 

is wound many thousand turns of fine insulated copper 
wire constituting the secondary — or high-tension — circuit. 
About 4 volts are applied to the primary circuit and the 
current repeatedly broken and remade by means of the 
magnetism of the iron core attracting a small piece of iron 
mounted on a spring which carries the current. As the 
spring is attracted inwards it loses contact with a platinum 
point and so breaks the current. (To make the break the 
more sudden it is usual to put a condenser in parallel in the 
circuit.) This sudden rise and fall of current in the primary 
causes oscillatory currents in the secondary of a voltage 
which is higher than that in the primary in the ratio of the 
number of coils of wire in one to the number in the other. 
Owing to the effect of the magnetism in the iron core the 
current in the primary does not rise suddenly to its full 
value. It follows in fact the law 



V 

u=- 

where 



<Hr-( 1 -^ ( ) 



C= current in amperes. 
V= volt age in volts. 
R= resistance in ohms. 
L =self -induction in henries. 
t=time in seconds. 

e=base of Naperian logarithms or 2*7183. 
The unit of self-induction is the henry. If S be the rate 
at which the current changes in amperes per second, the 
back electro motive force produced —L xS. One henry is 
also defined to be the self-induction of a coil in which, if 
the current increase at the rate of one ampere per second, 
the back E.M.F. produced is exactly one volt. 

As an illustration of the effect of the above law of rise 
of the current, take the case of a coil in which B=500; 
£=5-5 and 7 = 50,000. Then the final and steady value 

of the current is clearly - or 100 amperes. This 

500 * 

current grows up from zero and it is of interest to calculate 
how long it will take 90 amperes to be the current flowing, 

T 



274 THE INTERNAL COMBUSTION ENGINE 



90 = 100(1—^ o^oT 

or E o : oii_ 1 _ o.9=0-10 

so that £=about o 1 - second. 
The current therefore rises by no means instantaneously 
and this leads to the " make " of the primary current, 
producing a much less vigorous spark in the secondary 
than does the " break." The break is almost instantaneous ; 
the only thing that tends to prevent it being so, is the energy 
of rush of the primary current which jumps over the gap 
in its earlier stages. The energy stored up in the flowing 
current is equal to \LC Z , and it is to provide a convenient 
swamp to absorb this suddenly released energy that the 
condenser is provided. 




Fig 



96. — Diagram showing mode of working of high-tension ignition with 
coil and accumulator. A, Accumulator. B, Induction coil. C, 
Contact breaker. D, Trembler. E, Commutator on end of cam shaft, 
for closing circuit at right moment by bringing metal segment F against 
the brush G. H, Condenser, to make break of current sudden. I, 
Ignition plug in cylinder. The other end of the secondary winding 
is earthed. 



126. High-Tension Coil Ignition. — The induction coil is 
supplied with a low-tension current obtained from either 



OIL AND PETROL ENGINES 



275 



batteries, accumulator cells or a magneto. In any case 
the principle of working is the same. The spark gap is 
placed in the cylinder as shown in Figs. 96, 97, 98, and 99. 




Fig. 97. — Coil and accumulator Ignition, for a four-cylinder engine, with 
separate coils for each cylinder. A, Accumulator. B, Coils each 
with its own condenser and contact maker. E, Commutator for dis- 
tributing current to the various cylinders at the right moment. /, 
sparking plugs. 

A rotating contact kept at a speed proportional to that of 
the engine and called a distributor distributes the current 
to each cylinder just as it is needed. What happens there- 



¥ V k v 

• tiii tiii tiii 




i I l 

Fig. 98. — The arrangement shown in Fig. 97, except that one trembler 
serves all the coils. This saves having to adjust each trembler until 
all are working at same frequency. C is the common contact maker. 
and N are switches for cutting out coils when neeessarx . 

fore is this. The trembling blade on the coil — called the 
trembler — vibrates very rapidly and produces a shower of 
sparks in the secondary (one spark corresponding to each 



276 THE INTERNAL COMBUSTION ENGINE 

break of current in the primary). During each contact 
about a dozen sparks or more may pass. One good spark 
would be enough and therefore a modification of this 
method is sometimes employed. Instead of a trembler 
actuated by the magnetism of the iron core of the coil, 
the current in the primary circuit is made and broken by 
the action of the engine. A mechanical make-and-break is 



^AXX 




Fig. 99. — The arrangement of Figs. 97 and 98, except that the secondary 
current is distributed directly, so enabling only one coil to be used 
for all four cylinders. The disadvantage is that the insulation is 
more difficult to ensure. 



fitted to the half-speed shaft of the engine so as to produce 
one spark only in the cylinder. It is possible to ring the 
changes on this form of ignition so as to produce a great 
many varieties, although the differences between them are 
hardly fundamental. Illustrations, reproduced from Mr. 
Strickland's useful book, are shown of several such methods 
(see 96, 97, 98, and 99), and the letterpress at the foot of 
each will suffice to show their differences. 

127. The Lodge (Sir Oliver Lodge) system of ignition is 
just the ordinary coil and accumulator ignition in which the 
high-tension current instead of being passed direct to the 
sparking plugs is made to charge up the innner coatings of 
two Ley den jars. When the jars are " full " an external 
spark gap placed in parallel with the jars breaks down and 
a spark passes. This sudden release of the electric charges 
on the inner coats of the Leyden jars causes such a rush of 
current from the outer coating of one Leyden jar to the 
other and such a violent oscillation to and fro of the current 



OIL AND PETROL ENGINES 



277 



afterwards that nothing will 
stand in its path. It 
breaks through oil films, 
soot, deposit of all kind, 
water or anything else that 
there may be on the ignition 
points ; owing to its high 
frequency it also tends to 
take straight direct courses, 
and there is little dis- 
position on its part to seek 
any short circuit of a tor- 
tuous kind which may 
happen to be in existence. 

Fig. 100 shows diagram- 
matically the arrangements 
of the high-tension circuit. 
The low-tension circuit is 
of the customary form, 
except that the trembler 

shown in Fig. 101 is of an extra sensitive form. The dis- 
tributor is placed in the high-tension circuit. The makers 
of this ignition system claim that owing to the adjustments 
made no possible error in the time of firing can arise which 
exceeds 3 T) \ y T) th part of a second. Also that in virtue of 
the nature of the spark the system is particularly suitable 
for use when the fuel used is paraffin or any other heavy 
oil which may cause carbonaceous deposit on the ignition 
plugs. 

128. We now come to magneto 




Fig. 



B Spai k 



100. — The Lodge Ignition System. 
Diasram of H.T. circuits. 



Set Screw 
Brass Bridge im mW mi Locking Screw 
Brass Blade 



J 



Platinum Points 



FlG. 101. 




in nil 



•W 



V 



Iron Core 
Rubber Stop 



adge Sensit Lve 



mbk 



ignition. It may be 
either high tension or 
low tension. No coil 
is used and no bat- 
teries or cells are 
wanted. The low- 
tension method pro- 
ceeds on the principle 
that when a current 
is flowing it lias 



278 THE INTERNAL COMBUSTION ENGINE 



energy of motion equal to 
\LC' Z (analogous to kinetic 
energy, Jrav 2 ), and that if L, 
the self-induction, is made very 
great and C, the current, as 
great as convenient, the energy 
stored up is so considerable that 
a large or " fat " spark is 
caused to occur when the circuit 
is suddenly broken. A low-ten- 
sion magneto, or electric gener- 
i ezL ' \m ator, is designed so as to cause 

such a current to be passing at 
the moment when ignition is 
desired to occur and, at the same 
instant, the circuit is mechani- 
cally broken in the cylinder and 

Fig. 102.— Low-tension magneto a Spark passes. Fig. 102 shows 

ignition, X and IF, magneto diagrammatically the wiring for 

machine shown diagrammati- ° \ ° 

caUy. A, Spark plug. C, this system and the sparking plug 
Contact point where circuit is use( }. A larger view of such a 

closed and broken. B, Lever . . 

worked by rod running on plug IS Seen m Fig. 103. 

cam, e. E Cam on half-time ^he high-tension magneto con- 

shait. At the moment when . ° . , . , 

the magneto is passing its sists ot a magneto m which a 
maximum current around the numD er of turns of fine wire are 

circuit, the cam causes the cir- 




cuit to be broken at C, so pro 
ducing a spark at that point. 



wound on to the arm- 
ature so as to act as 
a secondary circuit. 
The effect is that 
very high voltage 
currents are pro- 
duced and the cur- 
rent can be led to 
sparking plugs of the 
ordinary type in the 
cylinders. This has 




Fig. 103. — Typical low tension magneto 
spark plug. It will be noticed that this 
system of ignition requires moving con- 
tacts in the cylinder, which the high tension 
system does not. 



OIL AND PETROL ENGINES 2?9 

the advantage that no moving parts need to be introduced 
into the cylinder in order to produce a spark. The 
voltage is so high that a " safety valve " spark-gap 
is usually fitted in parallel near the magneto in order to 
allow any unduly high voltage current which may be produced 
to pass across it. The spark in this gap is, of course, of 
no use except to act as a safety valve or bye-pass. 

129. The Simms-Bosch High-Tension Magneto System of 
Ignition. — In this system the current is generated by a 
shuttle armature which rotates between the poles of three 
pairs of very strong steel magnets. The rotation of this 
armature in the strong magnetic field results in the induc- 
tion in its winding of an electrical current which is utilized for 
the purpose of ignition. The armature is wound in two parts, 



Fig. 104. — Outside view of Bosch H.T. Magneto. (A low-tension magneto 
is of generally similar shape.) 

of which one is a primary winding, consisting of few turns 
of heavy wire, and the other a secondary winding, consisting 
of many turns of fine wire. The effect is that a high-tension 
current is given off by the armature, as the design practically 
amounts to the inclusion in the armature of the windings of 
an induction coil. An outside view of this magneto is shown 
in Fig, 104, and in the appendix to this chapter a full descrip- 
tion is given of its manner of working. 



280 THE INTERNAL COMBUSTION ENGINE 

130. Timing of Ignition. — One of the most careful adjust- 
ments of the ignition is its timing. That is to say, the 
regulation of the moment of sparking in the cylinder. If 
the spark is late the piston has moved part of its outward 
journey, with the consequence that the effective working 
stroke is lessened and the mean pressure is much lower than 
it should be. If the spark is too early, so that the gases 
are still being compressed when the spark comes, then 
there is a knock in the bearing when the explosion occurs. 
Normally the spark should occur just as the piston is at 
the top of its stroke, although since ignition takes a 
fraction of a second to spread throughout the mass 
of the gas it is necessary when the engine is running 
fast to time the spark to occur a little before the dead 
centre so that maximum pressure is reached when the 
piston is just beginning its stroke. Engine speed is, how- 
ever, not the only consideration affecting the timing ; 
when running with weak mixtures the ignition takes longer 
than with rich mixtures so that to use a weak mixture it is 
necessary to " advance " the spark, i.e. make it occur 
earlier. It follows, therefore, that in the ordinary running 
of a car the ignition requires as much attention as the 
throttle, if the engine is to work at highest efficiency. An 
additional complication arises when coils having tremblers 
are used with batteries or cells, as the speed of " trembling " 
being naturally independent of the speed of the engine, it 
follows that unless the ignition is advanced with increase 
of engine speed the sparking in the cylinder will occur 
later in the stroke. Cars — chiefly heavy ones — fitted with 
low-tension magneto ignition sometimes run on fixed 
ignition, i.e. the spark is designed to occur as near as 
possible always to the dead centre. This, however, may 
make it difficult to start the engine, since if the spark 
occurs before the instant of dead centre the pressure 
will produce a heavy blow on the starting handle and 
possibly break the driver's wrist. It is claimed some- 
times that with low-tension magneto ignition the ignition 
is advanced or retarded with the speed of the engine almost 
automatically, owing to a " fatter " spark being produced 



OIL AND PETROL ENGINES 281 

when the magneto is running fast and a presumed increase 
in speed of ignition. In some engines it is arranged that 
the governor shall control the ignition, and this plan is 
adopted in the Albion engine. 

131. The Albion engine is fitted with a low-tension 
magneto system of ignition and Fig. 105, which is repro- 
duced from a paper * by Mr. T. Blackwood Murray, shows 
very clearly the effect of mechanical lag in the movements of 
the mechanism at high speeds. As the speed increases so the 
firing point is advanced, until at 1,000 revolutions per minute 







A shows Lag oF Trip Gear. 






B " Apparent Advance. 

C " Actual Advance. * 


£0° 


<* 
^ 




■*j 


/ 




^ 


/ 




o 










. ~° 




S / 


40 r 




'*' Z^ 






^' ^^ 




c; 






^ 


^ "" ^^"^ 


20- 


-v^^-^-- _ 


- ~~ " ____^— -^^^ 




-> ? 






- -- 1 - ~~ 






— — """" 


0° 


— " 


— "" 



400 GOO 800 1000 R. P.M. 

Fig 105. 



it is adjusted so that the spark would occur no less than 
64° before the dead centre, but owing to a 14° lag on the 
part of the trip gear the spark only comes 50° early. In 
the Albion engine this control is left to the governor entirely. 
It will be noted that even at starting there is a real advance 
of about 20°, and this might be expected to make the 
starting rather difficult— indeed dangerous— to an unskilled 
driver, but from an experience of many Albion engines 
the author has never noticed any difficulty to arise from 
this cause. Mr. Murray's view of the matter is that : 

* "Some Details of Albion Motov Cars," road before Institution 
of Engineers and Shipbuilders in Scotland, 1907. 



282 THE INTERNAL COMBUSTION ENGINE 

" Owing to the fact that no spark can take place unless the 
crankshaft is rotating with a certain angular velocity, it 
is permissible to set the ignition to take place, even at start- 
ing, slightly before the dead centre, as this said velocity 
ensures the vis viva of the flywheel carrying the engine over 
the dead centre. This reduces the arc of ignition advance 
throughout which the magneto is called upon to generate 
an effective spark, and enables one to key the magneto 
once for all in a fixed position to the crankshaft, which 
might not be possible if too large an arc of advance were 
necessary." In other words, it has been sought to find a 
compromise between conflicting ideals of operation. 

In the Albion method, between fast and slow speed the 
point of ignition is moved through about 30° of the cir- 
cumference of the crank path. Thirty degrees at 1,000 
revolutions per minute corresponds to a time interval of 

=7r7T7r second, and this extra interval it is which allows 

12,000 200 

the ignited gases to be at their full pressure just as the 
highly speeded engine arrives at the dead centre. The 
ignition obviously affords a method of governing the engine, 
as if the spark be made to occur very late in the working 
stroke hardly any horse-power will be produced, but this plan 
is expensive in fuel and it is better to decrease the horse- 
power by decreasing the volume of mixture admitted to the 
cylinder. Here we have written rather of the ignition as 
affecting petrol engines than as affecting gas engines, large or 
small. The reason is that a petrol engine, especially as used 
on a car, is far more sensitive to such changes and makes 
a much better exploring instrument than a gas engine. But 
what applies to the one applies also to the other. Magneto 
ignition is becoming more and more common on gas engines, 
especially low-tension ignition, and what is still more 
striking is to see that gas engine builders are taking a leaf 
out of the books of the petrol engine builder and making 
the ignition variable so that it may be adjusted to starting 
conditions and sometimes even to exceptional running 
conditions. 

132. In a paper read by Dr. Watson before the Royal 



OIL AND PETROL ENGINES 283 

Automobile Club an interesting account was given of 
certain experiments undertaken to ascertain the character 
of the spark in relation to power. The engine used was a 
two-cylinder one, 3-5 in. x4 in., with mechanically operated 
valves. The sparking plug was screwed into the cap 
used to close the hole over the inlet valve, the spark points 
being well inside a recess in this cap. The whole of the 
experiments were made on one cylinder only, the other 
being operated with a trembler coil and battery. The 
speed was 950/1,000 revolutions per minute. It has often 
been claimed that a " f at " spark improves the running, 
and that this was due either to quicker ignition of the 
charger or to more regular firing. Experiments with a 
trembler coil showed that although the weakening of the 
current was found to reduce the mean pressure, yet this 
could be brought back to its original value by advancing 
the spark. The result of this series of experiments was 
to lead Dr. Watson to the following conclusions — 

1. As far as a petrol engine of the type used is concerned, the 
character of the spark which ignites the charge has no appreciable 
influence on the power developed. 

2. With a trembler coil the time at which the spark occurs is 
liable to vary greatly, and on this account the power developed 
may be considerably reduced. 

3. The variation in the time of firing obtained with trembler 
coils is different for different coils, and hence a multi-cylinder engine 
in which a separate coil is used for each cylinder is unlikely to 
develop its maximum power, particularly at high speeds ; the reason 
being that although the tremblers of the coils may possibly be so 
adjusted for some particular voltage that each cylinder fires at the 
same point of the stroke, yet this adjustment will no longer be true 
if the voltage of the battery alters, particularly if it falls much 
below the value for which the tremblers were adjusted. 

4. When a single coil is used in combination with a high-tension 
distributor, it is of very great importance that the current in the 
primary should never be allowed to fall to a value near the critical 
value for the particular coil. In this connexion it may be mentioned 
that, in Dr. Watson's experience, when the trembler is so adjusted 
for any given voltage of the battery, i.e. for a given current, that 
the note produced is very clear and "pure," then a very slight 
decrease in current, due to a small fall in the voltage o\' the battery, 
will cause the timing tO be defective, owing to (lie region of the 
critical current being approached. Hence, with the normal current 
passing— i.e. with the battery fully charged it is advisable to adjust 
the trembler so as to give a, somewhat harsh anil shrill sound, tor 



284 THE INTERNAL COMBUSTION ENGINE 

then the current may be considerably reduced before the critical 
value is reached. 

5. When selecting a coil, regularity in the working of the trembler 
for considerable variation in the current passing in the primary 
is of more importance than length or fatness of spark. Further, a 
coil taking a small current is to be preferred to one taking a large 
current, since trouble with the adjustment of the trembler blade 
will be decreased, owing to the reduced sparking at the platinum 
points with a small current. 

6. Except for the fact that the engine cannot be started on the 
switch, the plain coil with a rapid break on the two-to-one shaft 




Fig. 106. — Indicator Cards obtained by Dr. Watson. 

seems preferable to a trembler coil, since over a very large range 
of current — in fact, whenever the current is large enough to cause 
the passage of a spark in the cylinder — the timing is exactly the 
same. The advantage of the trembler might be retained by using 
a switch, so that after the engine is started the trembler can be cut 
out, allowing the coil to act as a plain coil, a second condenser being 
provided. 

The two diagrams shown in Fig. 106, obtained by Dr. Watson, 
illustrate the advantage, so far as economy is concerned, of 
advancing the spark more than usual when employing a very 
weak mixture — that is, when driving with the extra air valve as 
far open as possible. The lower figure is that obtained when 
the spark is as much advanced as is advisable when using a full 
mixture, and the i.h.p. at 1,000 revolutions was 2-36. In the 
upper figure the spark has been considerably further ad- 
vanced, so as to allow for the slow burning of a weak mixture, 
and as a result the i.h.p. at 1,000 is 2-76, an increase of nearly 
17 per cent, in power, the consumption of petrol remaining the 
same. 






OIL AND PETROL ENGINES 285 

133. Mr. J. A. Davenport has also carried out some 
experiments at the East London College.* The only things 
allowed to vary were the high-tension spark gap and the 
voltage of the cells. One set of experiments was carried 
out at two volts and one at four volts in the cells, and the 
experimenter found to his surprise that the engine ran as 
well on two volts as four. In his own words, his conclusions 
were that " as far as the tests go, they show that for the 
coil used, the best spark gap is about 0*030 in. Further, 
since the current taken at two volts is half that taken at 
four volts, the cells will run four times as long in parallel 
as they will run in series ; and that without injuriously 
affecting the consumption. Again, as the engine on a long 
run is slightly better at two than at four volts, everything 
is in favour of the use of two volts, instead of four volts 
as used in the standard practice." The engine used was a 
4 h.p. one and the coil was of the non-trembler variety 
having a make-and-break on the half-speed shaft. Mr. 
Davenport's conclusions agree generally with Dr. Watson's 
as supporting the proposition that it is the correct timing 
of the spark rather than its " fatness " which is of impor- 
tance. On the other hand, if conditions favour a " fat " 
spark probably one will take place even under circum- 
stances which might prevent any " thin " spark passing at 
all. 

134. Appendix. 

Description and Working of the Simms-Bosch High-Tension 

Magneto. 

Primary Winding. — The end of the primary winding is connected 
to the brass plate 1. In the centre of this disc is screwed the fasten- 
ing screw 2, which serves, in the first place, for holding the contact 
breaker in its place, and, in the second, for conducting the primary 
current to the platinum screw block 3, of the contact breaker. 
Screw 2 and screw block 3 are insulated from the contact-breaker 
disc 4, which is metallically connected with the armature core. The 
platinum screw 5 is arranged in the screw block 3. Tressed against 
this platinum screw by means of a spring 6 is the contact-breaker 
lever 7, which is connected to the armature core, and therefore with 
the beginning of the primary winding. The primary winding is 

* Engineering, February 22, 1907. 



286 THE INTERNAL COMBUSTION ENGINE 

therefore short circuited as long as lever 7 is in contact with platinum 
screw 5. The circuit is interrupted when the lever is rocked. A 
condenser 8 is connected in parallel with the gap thus formed. 

Secondary Winding. — The beginning of the secondary winding is 
connected to the end of the primary, so that the latter is a direct 
continuation of the former. The end of the secondary winding 
leads to the slip ring 9, on which slides a carbon brush 10 which is 




Fig. 107. — Arrangement of Bosch H.T. Magneto. 



insulated from the magneto frame by means of the carbon holder 11. 
From the brush 10 the secondary current is conducted to the con- 
necting bridge 12, fitted with a central carbon brush 13, and through 
the rotating distributor piece 14, which carries a radial contact 
carbon 15, to the distributor disc 16. 

In the distributor disc 16 are embedded metal segments 17, of 
which there are three in the magneto of type " D 3," four in type 
" D 4 " and six in type " D 6." During the rotation of the contact 
carbon 15, the latter makes contact with the respective segments, and 
always connects the secondary current with one of the contacts. 
Connected to the segments are sockets which serve for the reception 
of the contact plugs 18. These plugs serve as terminals for the 
cables leading to the sparking plugs of the individual cylinders. 

From the end of the secondary winding, the high tension current 
is led to the respective cylinders, which are fixed alternately, then 
returns through the motor frame and armature core to the primary 
winding, and back to the beginning of the secondary winding. 

Method of Operation. — The rotation of the armature in the mag- 
netic field generates an alternating current in the armature winding, 
which twice attains its maximum during each revolution, the two 
maximums being 180° apart. An ignition may therefore be pro- 
duced for each 180° rotation of the armature. 



OIL AND PETKOL ENGINES 



287 



The tension of the current generated by the rotation of the arma- 
ture is increased by short circuiting and opening the primary circuit 
through the contact breaker at the proper moment. At the moment 
the circuit is opened or interrupted, an arc light is formed at the 
sparking plug. As, however, the arc is only produced when the 
armature is in a certain position, which position must correspond 
to a definite position of the 
piston in the motor, it is neces- 
sary, that the armature of the 
magneto be driven positively 
from the motor. 

Speed of Rotation. — The 
speed at which the magneto 
must be driven depends upon 
the number of cylinders. 

In type " D 3," for instance, 
which is designed for three - 
cylinder motors, the armature 
must be run at a speed corre- 
sponding to three-quarters of 
the speed of the crankshaft. 
Type " D 4," for four-cylinder 
motors, must be run at the 
same speed as the motor, and 
" D 6," for six-cylinder motors, 
must be run at one and a half 
times the speed of the motor. 

Distribution of Current. — The 
disc connected to the distri- 
butor brush 15 and which re- 
volves the latter is geared in 
the different types from the 
armature shaft at such a ratio 
as to rotate the contact brush 
at the speed of the cam shaft 
of the motor. 

Contact Breaker. — The contact breaker is keyed to the armature 
shaft and is fastened by means of screw 2. It can be easily removed. 
Upon replacing the contact breaker, care should be taken that the 
key mentioned is placed in its key-way, and that screw 2 is well 
tightened up. 

The short circuiting and interrupting of the primary circuit is 
effected twice during each revolution of the armature by moans of 
the contact-breaker lever 7 on one hand, and the fibre rollers 19 on 
the other. As long as lever 7 is pressed against contact screw 5, 
the primary circuit is short circuited, the rocking of the lever through 
the fibre rollers 19 effect the break of the primary circuit, and at 
the same moment the ignition takes place The movement oi the 
lever 7 should not be more than -5 nun. and may be adjusted by 
means of screw 5. 

Jn order to protect the insulation of the armature and oi the 




Fig. 108. 



288 THE INTERNAL COMBUSTION ENGINE 

current-carrying parts of the apparatus against excessive voltages, 
a safety spark gap is arranged on the dust cover 21. The current 

3 




rfarffllt] 









will pass through this gap in case a cable is taken off while the 
magneto is in operation, or if it should accidentally become dis- 



OIL AND PETROL ENGINES 289 

connected. The discharges, however, should not pass through the 
safety gap for any length of time, especially not when the motor 
is equipped with a second system of ignition, and it is in such a case 
absolutely necessary to short-circuit the primary winding as above 
described, and thereby switch off the ignition. 

Timing of the Ignition. 

The variation in the time of ignition is effected by causing the 
interruption of the primary circuit to take place earlier or later. 
For this purpose the timing lever 20 is arranged to be either advanced 
or retarded which produces either an earlier or a later interruption, 
and consequently an earlier or a later ignition. A variation of 
40° on the armature spindle is thus possible, which is equal to over 
50° on the shaft of the motor for three-cylinder motors, about 40° 
for four-cylinder motors and about 27° for six-cylinder motors. 

PROBLEMS. 

1. Describe, with sketches, a gas or oil engine cylinder, 
showing valves and piston. (B. of E., 1907.) 

2. Describe, with sketches, only one of the following — 
A steam or gas engine governor, and how it regulates. 

A spirit or oil engine for a motor car, showing how it 
drives the car and how it works. 

(B. of E., 1901.) 

3. Describe, with sketches, one, and only one, of the 
following — 

Any form of governor. 

An engine used on any kind of motor car. 

4. Describe, with sketches, the carburettor of a petrol 
engine. (B. of E., 1907.) 

5. The area of a petrol engine diagram is (using the plani- 
meter which subtracts and adds properly) 4-12 square 
inches, and its length (parallel to the atmospheric line) is 
3-85 in. ; what is the average breadth of the figure ? If 
1 in. pressure represents 70 lb. per square inch, what is the 
average effective pressure ? The piston is 3-5 in. in diameter 
with a stroke of 4 in. What is the work done in one cycle ? 
If there are 800 cycles per minute, what is the horse-power ? 

Ana. 107 ins., 749 lb. /in. 2 , 2403 ft, lb., and 582 lip. 

6. Describe, with sketches, the working of a good oil 
engine. How is the whole energy of the charge disposed 
of ? (Mech. Sc. Tripos. Part 1 . 1 898.) 

U 



290 THE INTERNAL COMBUSTION ENGINE 

7. Describe the method of balancing employed in any 
motor car engine known to you. 

8. Describe carefully, with indicator diagram and sketches, 
the action of the oil in its passage through the oil engine. 
What is understood by (1) after-burning, (2) scavenging ? 
What is the method adopted for governing the engine ? 

(Cambridge B.A. Degree, Old Regulations, 1904.) 

9. Problem on carburettor design. Show how the calcula- 
tions given in this chapter would be affected by the introduc- 
tion of terms representing friction to passage of air and petrol. 
If, as is stated, the frictional resistance of petrol flowing 
in a pipe varies largely with temperature, show that definite 
calculation becomes far more difficult. (Experiment, how- 
ever, would be simple, and students are recommended to 
undertake it as a piece of research work which would be 
important and valuable.) 



CHAPTER IX 

Petrol Engine Efficiency and Rating 

Efficiency Tests under Various Conditions — Effect ~of 
Cylinder Dimensions on Efficiency — Engine Rating — 
R.A.C. Rule — Callendar Rule — Composition of Exhaust 
Gases as Related to Efficiency — Road and Air Resist- 
ance — " Gross-ton-miles-per-gallon " Measurement. 

135. Efficiency Tests on Petrol Motors. — Among the most 
searching tests that have been carried out on petrol motors 
are those undertaken in the Engineering Laboratory at Cam- 
bridge under the supervision and guidance of Professor 
Hopkinson. 

In one set of such tests * the engine used was a 16/20 
h.p. Daimler four-cylinder engine capable of running at 
250 to 1,400 revs, per min. Other particulars were — 

Total volume of one cylinder with 

piston on out centre . . . 0*04 cu. ft. 
Volume of compression space . 0-0104 cu. ft. 
Compression ratio .... 3-85 
Diameter of cylinder . . . . 3*56 inches =90 mm. 
Length of stroke 5-11 inches = 130 mm. 

The type of indicator used was one invented by Professor 
Hopkinson. It was of the piston type, the piston being 
forced against the mid point of a piece of spring steel held 
at both ends, the deflection of which rotated a mirror 
through an angle, and so moved a spot of light on a screen. 
The mirror mechanism was rocked at the same time in a 
perpendicular direction, corresponding to the motion of the 
piston. These two motions combined to give the usual 

* Engineering, February S. 1007. 

291 



292 THE INTERNAL COMBUSTION ENGINE 

indicator diagram, which was then thrown on to a screen or 
photographed. This is an adaptation of a principle first 
used by Professor Perry many years ago. 

The tests involved three sets of measurements — (1) en- 
gine losses, (2) b.h.p., and (3) fuel consumption. From (1) 
and (2) the i.h.p. could be obtained, and therefore the 
mechanical efficiency. The tests were run with the car- 
burettor as fitted by the engine builders, and it must not 
therefore be taken that the engine was of necessity 
adjusted to give maximum power or efficiency. 

The results of the tests are shown in Fig. 110 in which 
curves are given for the i.h.p., b.h.p., the mean effective 
pressure and the torque on the crankshaft. It will be seen 
that the m.e.p. is nearly constant and equal to about 85 
lb. /in 2 . The mechanical efficiency varies from 85 to 75 per 
cent. — falling slowly as the speed exceeds 600 revs, per min. 
The petrol used had a thermal efficiency on the lower scale of 
17,000 B.T.U. per lb. and on this basis the following table of 
thermal efficiencies was calculated — 



Speed. 


Petrol Consumption (Pounds). 


Thermal 


Efficiency. 


Revs, per 


Per I.H.P. 


Per B.H.P. 


Per 1,000 


On 


On 


Minute. 


Hour. 


Hour. 


Revs. 


I.H.P. 


B.H.P. 


400 


0-78 


0-9 


0-30 


18-6 


161 


400 


0-75 


0-87 


0-28 


19-3 


16-6 


600 


0-685 


0-81 


0-26 


21 


17-9 


600 


0-655 


0-77 


0-24 


22 


18-8 


800 


— 


— - 


0-24 


— 


- — 


1,000 


0-6 


0-75 


0-22 


24-2 


19-3 


1,000 


0-6 


0-75 


0-206 


24-2 


19-3 


1,100 


0-59 


0-785 


0-202 


24-6 


18-4 


1,225 


(0-65) 


0-94 


0-22 


(22-3) 


154 



Note. — At speeds 400, 600, and 1,000, two tests are given to 
show the range of variation. At 1,225 the indicated horse-power 
is uncertain, as no direct measurement of loss was made at that 
speed. 



The thermal efficiency rises considerably with increase of 
speed — due no doubt in part to there being less time for the 



PETROL ENGINE EFFICIENCY AND RATING 293 



cold walls to cool the explosive mixture, but in view of the 
variability of the composition of mixture passed by the 
carburettor (of the usual jet type) it is not safe to build any 
deductions on these measurements, although it is very useful 
to have them recorded. An interesting measurement in 
addition to the above was that of the pressure in the in- 
duction pipe. With the throttle wide open and a speed of 



H P 
2 4 



20 



Piston Speed. 























/ 


























/ 


S 
























& 


l.H.P. 
























~y^ 














■5-^T — 
































T0C3, 




















/ 


























/ 


'A 


B.H.fi 


> 






















/ 
























/ 


{/ 


























// 


/ 
























/ 


/ 
























/ 


/ 



























ZOO 



400 



1000 



1200 



120 



no 



100 



80 



70 



GO 



2 



10 



1400 



BOO 800 

Revs, per Minute. 
Fig. 110. — Professor Hopkinson's tests on 16/20 H.P. Daimler Engine. 



1,000 r.p.m. this pressure was about 1J lb. /in 2 , below 
atmospheric pressure. With the speed reduced to 400 
r.p.m. this pressure was less than J lb. /in 2 , below atmo- 
sphere, showing that either the carburettor was supplying a 
richer mixture at the lower speeds, or that only § as much 
aii- is taken per revolution at 1,000 as at 400 revs. The 
design of the average carburettor provides for over-supply 
of extra air at high speeds rather than under-supply, and 
the mixture probably was richer at 400 than 1,000 revs. 



294 THE INTERNAL COMBUSTION ENGINE 

Both the causes suggested were therefore in all probability 
at work. 

136. Some tests * to discover the relation between the rate 
of consumption of petrol per b.h.p., and the character of the 
spark employed were undertaken at the Central Technical 
College by Messrs. Topham and Tisdall. A small single 
cylinder De Dion engine was used of 66 mm. bore, and 70 mm. 
stroke. The proportion of air to petrol varied with atmo- 
spheric conditions and was adjusted for each test so as to give 
a maximum power development, the throttle was kept 
fully open, and the air supply alone was varied. The engine 
was run at full normal speed. The chief results obtained 
were as follows — 



Source of Spark (high tension). Petro1 Con « u i?P tio T n ^ g alls P er 

B.H.P. Hour. 



Accumulator with no external gap 
Magneto with external gap . 
Accumulator with external gap 



0-208 to 0-187 
0-289 to 0-249 
0-226 to 0-24 



Not much can be gained from these measurements, 
though it would seem that different forms of high-tension 
ignition have a generally similar effect on the rate of fuel 
consumption, the accumulator having a slight advantage. 
The result of using an external spark gap in the high- 
tension circuit is worth recording. The plug insulation 
resistance was over 1,000 megohms cold, but after 
being used for running the engine for five minutes, it 
fell to about 6 megohms. At a dull red the resistance 
fell further to 2 megohms, and eventually sparking 
ceased entirely. On the introduction of an external air 
gap in the circuit, however, sparking was resumed, even 
when the plug was made bright red hot, and the resistance 
had sunk as low as 800,000 ohms. The experimenters con- 
sidered that this showed that leakage through the porcelain of 
the plug might become quite large enough to stop sparking 
entirely, and that the effect of the introduction of the 
external air gap was to prevent the application of the 



Engineering, December 28, 1906. 



PETROL ENGINE EFFICIENCY AND RATING 295 

voltage across the sparking points until the instant at which 
current began to flow in the high-tension circuit, the spark 
then being of the nature of a condenser discharge, and the 
leakage being considerably reduced in quantity. 

137. The Effect of Cylinder Dimensions on Efficiency.— A 
good many attempts have been made to produce a working 
theory of cylinder dimensions, particularly of cylinder 
diameters, as affecting the economy and power of internal 
combustion engines. Most of the attempts have owed 
their origin to the competitive trials of motor cars in which 
the various vehicles are classed according to their horse-power 
and which therefore require that the figures given should be 
properly comparable. If no such precautions are taken, it 
becomes possible, for instance, for the owner of a high 
powered car to enter it as of small h.p., and so give it an un- 
fair advantage, say in hill- climbing, over a car the power of 
which was really this figure. For large internal combustion 
engines the existence of such a rule is not of vital importance, 
since the comparison of one engine and another depends 
upon so many other factors, and moreover organized com- 
petitions are unknown. 

This subject has been discussed by Professor Callendar, 
in a paper before the Society of Automobile Engineers * 
under the title of " The Effect of Size on the Thermal 
Efficiency of Motors." Cylinders were considered of 
as great diameter as 14 inches which, although not very 
large for a gas engine, was well outside the range of motor 
car cylinders. This makes the paper the more valuable in 
the general sense, even if from the strictly motor car point 
of view it may appear that advantage would have been 
gained had it been possible to build up the theory upon 
engine trials with cylinders of more nearly the customary 
motor car size. As it is, however, the paper presents a 
general theory not only applicable to motor cars but to 
larger engines also and few writers on the subject have 
exhibited so skilful a scientific treatment, or so able a grasp 
of the essential problems presented. For details, reference 
must of course be made to the actual paper, but a brief 

* May 8, 1007. 



296 THE INTERNAL COMBUSTION ENGINE 

description and discussion may with advantage be given 
here. 

138. It is well known, and it has been discussed in an 
earlier chapter, that the " air standard " of efficiency is 
much higher than the efficiencies obtained in practice, and 
that the ratio of the latter to the former is commonly about 
60 per cent. This means a deficit of 40 per cent, owing to 
some cause or other. What is this cause ? The answer is, 
first, that the " air standard " of efficiency is a far higher one 
than any actual engine can ever achieve, owing to the fact 
that whereas the value of y assumed in the " air standard " 
equation is 1-40, its average value for the actual cylinder 
gases at working temperatures, taking the increase of 
specific heat into account, would be more nearly 1-3. This 
alone accounts for about 20 per cent, of the 40 per cent, 
apparently lost, and the remaining 20 per cent, is due to 
various heat losses such as jacket loss, exhaust loss, radiation 
loss, etc. In general, therefore, it would appear that the 40 
per cent, loss is divided about equally between the two, 
but a more exact inquiry into the matter has shown 
Professor Callendar that it would be more correct to put 
the unavoidable apparent loss due to the properties of 
the gases down as 25 per cent., and the remaining 15 per 
cent, to the loss of efficiency owing to heat losses during the 
operations of the cycle. It is quite clear that, as the 
volume of gas in a cylinder will be proportional to the 
cube of the dimensions, and the surface of the cooling 
walls proportional only to the square of the dimensions, 
doubling the size of an engine will halve the heat losses 
due to cooling. The fact that the larger engine will pro- 
bably not run at so high a speed has very little effect on this 
conclusion, as although the time the gases will have to ccol 
will increase with diminishing speed, yet the diminished 
speed will lead to diminished scrubbing of the cylinder walls 
by the molecules of the gases, and so leave matters much 
where they were. It may therefore be said that the loss 

of efficiency due to cooling will be qc — where D is the cylin- 
der diameter. Some losses, however, such as the exhaust 



PETEOL ENGINE EFFICIENCY AND RATING 297 

loss, are not due to cooling, and what law of dimensions they 
will follow it is not easy a priori to say. Still, the most 

important loss has been shown to be proportional to— , and 
as an attempt at a working theory, there is no harm in group- 
ing the losses together and putting them proportional to — . 

This is what Professor Callendar does with several sets of 
engine trials. One set is based on his own experiments 
on an engine with a cylinder diameter of 2-36 inches, and 
the others are drawn from the Report of a Committee of the 
Institution of Civil Engineers. From the combined results 
on all four engines, he finds that the value of the constant a 
to be put in the expression — 

loss of efficiency = — 



actually comes out as 1*0 when D is measured in inches. So 
that loss of effic: 
to be drawn up. 



that loss of efficiency =— . This enables the following table 



Designation of engine. 
Diameter of cylinder, inches . 


C 
2-36 


L 

5-5 


R 
90 


X 

140 


Loss of efficiency ( = -jy) 


0-42 


018 


011 


007 


Resulting efficiency figure! 1 — -yr) 


0-58 


0-82 


0-89 


0-93 


Observed relative efficiency as 

compared with air standard . 
Ratio of last two lines .... 


0-44 
0-76 


0-61 

0-75 


0-65 
0-73 


0-69 
0-74 



139. The author understands that the value of the above 
constant a, viz. 1-0, was obtained by Professor Callendar 
mainly, if not entirely, from the above table. It was 
obviously necessary in accordance with the theory that the 
figures in the bottom line should come out the same, or as 
nearly the same as possible, and the value of a which was 
found most nearly to do this, happened to be 10 ; had the 



298 THE INTERNAL COMBUSTION ENGINE 

cylinder diameters been measured in millimetres the value of 
a would have been 25»4. 

In this way the relative efficiency of any engine can be 

written down as 0*75 ( 1 — — jand if the " air standard " 

efficiency for the degree of compression under consideration 
be called E, then the 

thermal efficiency =0-75^^1 — —J 

Had the " air standard " been a standard really applicable 
directly to gas engines, the figure 0*75 would have been unity, 
so further simplifying the formula. As it is the above equa- 
tion shows that even the largest engines cannot get nearer 
the " air standard " than 75 per cent. The establishment, 
even provisionally, of such a rule as this has an important 
influence in the consideration of the effect of dimensions on 
engine performance. 

140. The Royal Automobile Club published, in 1906, the 
following engine rating based on cylinder diameter : 

Nominal b.h.p. = 0-40.D 2 per cylinder, where D = cylinder 
diameter in inches. 

It has been found to give results which follow very nearly 
the ratings in general use in the motor world. It is equiva- 
lent to assuming that the piston speed is constant and equal 
to 1,000 feet per minute, and that the mean effective pres- 
sure on the piston during explosion is 67*2 lb. per square inch. 
It takes no account of changes in mean pressure owing to 
change in dimensions or in the degree of compression, nor 
for any change in piston speed owing to alterations in the 
ratio of stroke to bore or for other reasons. Professor 
Callendar points out that by altering the compression 
ratio from the 3-5 customary with petrol engines to 5-0 
about 20 per cent, more power could be obtained than the 
R.A.C. rating would give. 

The R.A.C. rating depends on the fact that with fixed 
piston speed and a constant value for the mean pressure the 
h.p. will vary with the area of the piston, i.e. vary as D 2 . But 
this assumes that all sizes of engines are equally efficient 



PETEOL" ENGINE EFFICIENCY AND RATING 299 



u l sujOZ P u e sayou/) japu/f/Cj jo jaqaaiBiQ 







US Q U) o 

<*) ^2 cm cn 
f Zs)sdqcui pue sajqaujmiy ui japu///Cj jc 



'SqSlUBIQ 



300 THE INTERNAL COMBUSTION ENGINE 

when worked under the same conditions, which is not the 
case. If D 2 be multiplied by the expression^ 1 — — ) as pro- 
portional to the efficiency, the result is to obtain the expres- 
sion D(D — 1) which therefore should be used in place of 
D 2 in the rating formula. In this way Professor Callendar 
suggests a " P.C. rating " (Petrol Consumption rating) of 

b.h.p. =5.(0-1) 

the figure 2 being the suitable constant. The P.C. ratings 
and R.A.C. ratings for a number of cylinder diameters are 
given in the following table — 



D. 


R.A.C. Rating 


P.C. Rating 


(per cylinder). 


(per cylinder). 


Inches. 


B.H.P. 


B.H.P. 


1 


0-40 


nil 


2 


1-6 


1-0 


3 


3-6 


30 


4 


6-4 


60 


5 


100 


100 


6 


14-4 


150 


8 


25-6 


28-0 


10 


40 


450 


20 


160 


190 



These two formulae are shown plotted in Fig. 111. The 
Callendar formula — (D — 1) gives the h.p. per cylinder; 
putting it in the same form as the R.A.C. formula it would 

b.™(D-H. 

The result of using the P.C. rating would be, to quote 
Professor Callendar : — " According to the R.A.C. formula, a 
four-cylinder engine with 2 inch bore and stroke (like the 
F.N. motor cycle engine), is rated at 6*4 h.p., and is equi- 
valent to a single-cylinder of 4 inches bore. According to 
my experiments the four-cylinder of 2 inch bore could not 
develop much more than 4 h.p. under ordinary conditions, 
and would stand no chance against the single-cylinder of 4 



PETROL ENGINE EFFICIENCY AND RATING 301 

inches bore. A four-cylinder of 3 ins. bore is equivalent 
to a single-cylinder of 6 in. bore by the A.C. rating, but 
according to the P.C. rating, the single -cylinder would have an 
advantage in point of power of 25 per cent. A two-cylinder 
of equal power on the A.C. rating would have an advantage 
of about 12 per cent, over the four-cylinder, and a six-cylin- 
der a disadvantage of about 10 per cent." Professor Cal- 
lendar also remarks : — " An obvious objection to the P.C. 
type of formula is that the b.h.p. of an engine of 1 in. bore 
and stroke would be zero. According to the R.A.C. rating it 
should be § h.p. It would no doubt be possible to get such 
an engine to run if very delicately made, but the effect of 
ignition lag would be serious at the normal speed of 6,000 
revolutions per minute, and I doubt whether it could be 
made to give as much as y 1 ^ h.p. on the brake." 

141. It may be remarked incidentally that small cylinders 
have a considerable advantage in h.p. per lb. of weight, as 
while the h.p. only increases according to the square of the 
piston diameter, the weight varies more nearly as the cube, 
and if made to the same drawings, but to different scales, 
would be actually as the cube. 

The P.C. rating of —D{ — 1) requires to be corrected for 

the possible variation of compression ratio, and this can be 
done by multiplying by an expression proportional to the 
" air standard " efficiency, and using a suitable constant. 
So corrected, the P.C. formula becomes : 

b.h.p. =2-5.0- (D— 1) 

or l-25ED(D— 1). 

This formula should therefore be used when it is desired 
to compare several engines of different sizes on the basis of 
their petrol consumption. Professor Callendar points out 
that according to his investigations the effect of increasing 
cylinder diameter from 2 in. to 4 in. is equivalent in efficiency 
improvement to increasing the compression ratio from 3 to 5. 
If would be very interesting to test this experimentally. 



302 THE INTERNAL COMBUSTION ENGINE 

One would have anticipated that such an increase in com- 
pression ratio would have had much the larger effect. 

Professor Callendar also deals with the modification that 
would be required to be made in his P.C. formulae if the 
ratio of stroke to bore became much more than unity, and 
he gives the modified formula as 

Where L— stroke and x—L — D. 
The basis of this correction is however somewhat un- 
certain, and as the correction itself becomes a very small 
one in practice, it need not usually be taken into account. 
Investigation is also made into the effect of a change in piston 
speed to which a change in the ratio of stroke to bore might 
naturally be expected to lead, and the following expression 
for the piston speed is suggested — 

Piston speed = 1,000 (l+- — \ feet per min. 

Such a formula gives a piston speed varying from 1,000 to 
1,100 as the ratio of stroke to bore increases from 1/1 to 4/3. 
In the case of hill climbing contests, it is desirable that a 
correction should be made so as to enable the true b.h.p. to 
be obtained from the cylinder dimensions and all that varies 
therewith. To this end, Professor Callendar establishes 
what he calls an H.C. rating in this form — 

H.C. rating b.h.p. =^(2)— 1)+— 

This of course may also be multiplied by a term proportional 
to E to correct for compression ratio variation. 

142. An interesting point is to find out what is the best 
compression ratio for minimum weight of engine per b.h.p. 
developed. 

If the weight cc D 3 (though in many engines D 2 ' 5 would be 
nearer the mark), and the h.p. cc D(D — 1) 
Then 

h.p. _D(D— 1) D—l 



weight D 3 D 2 

gives the following table : — 



which 



PETROL ENGINE EFFICIENCY AND RATING 303 



D :— 


1 


2 


3 


4 


5 


10 


D— 1 


:— 


0-25 


0-22 


0-19 


0-16 


0-09 



D 2 

showing that the greatest h.p. per lb. weight of motor would 
be obtained when D—2 inches, although D=S inches gives 
almost as good a result, and in cases where the weight is 
proportional to D 2 ' 5 the value D = 3 gives the best result. 
The weight will however be affected by any change in 
the compression ratio since with increasing compression 
the engine parts must be made heavier. Professor Cal- 
lendar makes an estimate of the effect of compression 
ratio thus : — " If we assume that the maximum pressure 
is nearly proportional to the compression pressure, and that 
the strength and weight of the motor and its gear must be 
proportional to the maximum stress, we may take the com- 
pression pressure as a measure of the weight. Taking the 
air-standard efficiency E as a measure of the mean pressure 
and the power, we find that the weight -f- power ratio is 
nearly twice as great for a compression ratio 5 as for a com- 
pression ratio 2, and diminishes with diminution of com- 
pression-ratio, reaching a minimum at r = l«9 if <y = l.40. 
The best value of the compression ratio depends on various 
conditions, but chiefly on the ratio of the load carried to 
the weight of the motor and its accessory gear. If the load 
is equal to the weight of the motor, the compression ratio 
should be rather less than 3. If the load is double the 
weight of the motor, the compression should be about 4*5. 
The above shows that in many cases where weight-saving is 
a primary consideration it may be desirable to reduce the 
compression considerably, and that even in automobiles 
there is no great advantage in increasing the compression 
ratio beyond 4*0, provided that lightness of construction and 
smoothness of running are duly considered in the design of 
the motor." These conclusions are of very great interest, 
though owing to the complexity of the phenomena it is not to 
be expected that they will command by any means universal 
assent. 

143. Exercise. — The great desideratum for high efficiency 



304 THE INTERNAL COMBUSTION ENGINE 

is large cylinder capacity in proportion to the internal 
cooling area of the cylinder. This may not make for 
" flexibility," as that is increased by the existence of 
" pockets" in the cylinder end, and "pockets" mean a 
considerable addition to cooling area. What ratio of 
cylinder diameter to length makes the ratio 

area of exposed surface inside cylinder in sq.ins. 
capacity of cylinder in cubic inches 
a minimum ? The answer to this question is the answer we 
want. 

Let the volume enclosed be V, and cylinder diameter be 
2a. 

V 

Then height = — =h 

ira 2 

The internal surface of cylinder (including the surface of 

piston face) 

= 2ira 2 -\-2ivah 

„, . t» ,. surface 2ir(a 2 4-ah) 

Therefore Ratio = — * ' L 

content V 

Substitute for h as above 

and Ratio =—-^(a 2 + ) 

V \ ira / 

To find when this is a minimum differentiate and equate to 
zero. 

Therefore 2a— =0. 

ira 2 

and 2a = — =h. 

ira 2 

Therefore diameter and height must be equal to produce the 
desired effect or the ratio of stroke to bore must be unity. 
This interesting result bears out actual practice inasmuch 
as in most engines the ratio of stroke to bore is always nearly 
unity. 

Suppose however that " pockets " are introduced, so 
that the surface is increased whilst the capacity is but 
slightly affected, what effect does this produce on the above 
calculation ? 



PETROL ENGINE EFFICIENCY AND RATING 305 

Surface now =27ra 2 -\-27rah-\-ca 2 where c is some constant 
={27r+c)a 2 +27rah 
y 
as before h — — very nearly 

ir a 



so that Ratio 



2 
surface _ {2ir-\-c)a 2 -\-2'wah 



volume V 



^I(27r+c)a 2 + 2 Zj 



V 

Differentiate and equate to zero. 

2V 
Therefore 2(2ir-\-c)a — ■—-. ■ 

a 2 



or 



V , 
v ' a 2 



So that Ratio A = 2 I± C = l +A 

2a 2ir 2ir 

so that ratio of stroke to bore will be greater than unity. 
— is of course the fraction of the piston area of which the 



pocket increases the internal area of the cylinder. If 
this ratio = J then — = 1-125, giving a relation which is 

quite close to that adopted in practice with engines which 
have " pockets." 

144. Composition of Exhaust Gases.— The efficiency of a 
petrol engine naturally depends on the degree to which com- 
bustion is complete. The exhaust gases should not, for 
good efficiency, contain any CO. All the carbon present 
should be burnt to C0 2 . Nor, if the proportion of air is 
closely adjusted, will there be any oxygen in the exhaust. 

It is difficult to write down the chemical formula in 
accordance with which the combustion of petrol takes place 
in an atmosphere of air, owing to the complex nature of the 
petrol molecule. An example of a chemical formula for a 
sample of petrol is 

41.86C 6 H 14 +6.48C 7 H 16 +C 5 H 12 . 

It is not easy to work equations containing a substance like 

x 



306 THE INTERNAL COMBUSTION ENGINE 

that. But since C 6 H 14 (or hexane) is by far the largest con- 
stituent it is interesting to write down the combustion equa- 
tion for hexane, and to look upon it as representing, roughly, 
what occurs with petrol. In any case no exact theory for 
petrol could be worked out as the composition varies so 
greatly between the different qualities used in practice. 
C 6 H 14 burns with 2 as follows — 

2C 6 H l4 +190 2 = 12C0 2 + 14H 2 

so that 21 volumes of mixture give 26 volumes of products, 
or, if the steam be condensed to water, 12 volumes. 

In actual working, ordinary air and not pure oxygen is 
used, so that there is nitrogen also to be considered. With 
19 volumes of oxygen, 19 x 3-76, or 71-3 volumes of nitrogen 
would be associated — making a total of 90-3 volumes of air. 
Each volume of hexane therefore requires 45-1 volumes of 
air for complete combustion and the equation can be 
therefore rewritten as — 

2C 6 H 14 +190 2 +71-3N 2 = 12COo + 14H 2 0+71-3N 2 . 
The right-hand side of this equation is exhaust products, and 
the composition by volume will be — if the volume of the 

12 
water be neglected — or 14*4 per cent, of C0 2 and 85*6 

per cent, of N 2 . If too little air were admitted some of the 
C0 2 would be reduced to CO, which being a poisonous gas 
is a very undesirable element in the exhaust ; moreover, it 
reduces the thermal value of the gas owing to a part of the 
carbon not being completely oxidized — a loss which has 
already been dealt with quantitatively in the chapter on 
suction producer gas. If too much air is admitted, free 
oxygen will appear in the exhaust. We have therefore the 
following three deductions : — 

(1) When oxygen occurs in the exhaust too much air 

has been admitted 

(2) Too little air leads to formation of the poisonous CO. 

(3) When neither CO nor 2 appear in the exhaust the air 

is in just the right proportion. 
It will now be the easier to understand some experiments 



PETKOL ENGINE EFFICIENCY AND RATING 307 

on the composition of exhaust gases made by Professor 
Bertram Hopkinson and Mr. L. G. Morse. The experiments 
were made in the engineering laboratories at Cambridge, 
on the Daimler engine already referred to. 

The speed was kept at 700/750 r.p.m., and a jet carbur- 
ettor of the usual sort was used. The throttle was kept open 
so that the suction never exceeded \ lb. pe,r sq. inch in the 
inlet pipe close to the inlet valves. Fuel used was Pratt's 
motor spirit; density 0-715 to 0-720; Calorific value 18,900 
B.T.U. (lower value). The exhaust gases were analysed by 
the ordinary volumetric methods, the C0 2 being absorbed by 
potash, the oxygen by pyrogallol, the CO by an acid solution 
of cuprous chloride, and the H 2 by palladianized asbestos. 

The following table shows the results recorded — 

Experiments made by Professor Bertram Hopkinson and 
Mr. L. G. Morse. 



Petrol consumption in lb. 

per 1,000 revs. 
Brake load at 43 in. 

radius. . . .lb. 
Thermal efficiency . 
C0 2 — measured . 
o 2 ■ „ . . . 

CO . . . 

H 2 „ .... 

N 2 by difference 
Total 2 , calculated 

from N 2 . . . . 
H 2 calculated. 



0181 


0191 


0197 


0-217 


0-250 


25 


27-5 


29-3 


29-4 


29-3 


0-244 


0-252 


0-261 


0-238 


0-204 


10-9 


12-8 


13-5 


10-6 


9-6 


3-6 


15 


0-2 


— 


— 


— 


— 


0-7 


5 


6-25 


— 


— 


— 


2 1 


2-65 


84 


84 


84 


81 


80 


22-4 


22-4 


22-4 


21-5 


21-3 


15-8 


16-2 


16-8 


16-8 


172 



0-293 

27 
0162 
6 

11-6 

8-7 
73 

19-4 
15-2 



145. From this it will be seen that when the CO and 2 
are at a minimum, the C0 2 is 13-5 per cent, and the N 2 84 
per cent., figures which are very close to those calculated 
above from the chemical formula. Moreover, it will be seen 
that it is at this point that the highest thermal efficiency 
(0-261) was recorded. This is best brought out when the 
points are plotted in a curve as in Fig. 112. 

Curve B (thermal efficiency) shows how quickly the 
thermal efficiency declines when CO begins to be pro- 
duced. Curve A (corresponding to b.h.p.) is of a very dif- 
ferent form, as although it is true that minimum production 



308 THE INTERNAL COMBUSTION ENGINE 



of CO corresponds to an output in h.p. very little less than 
the maximum, yet that maximum is found when the CO 
amounts to 0*7 per cent, and is very nearly maintained even 
when the proportion of CO rises to over 6 per cent. The 
ideal condition of working is obviously the apex of Curve B, 
but the corresponding point on Curve A is not a convenient 
one to work at. In all engineering work it is customary to 
m work, if possible, 

near to the middle 
of a curve which 
has a slow hump 
such as Curve A 
in order that small 
variations to right 
or left may not 
make much differ- 
ence. The ideal 
working point on 
Curve A is there- 
fore by no means 
the most conven- 
ient practical 
point, which in 
this case would 
corre s p o n d to 
about 5 per cent, 
of CO. This temp- 
tation has led 
engine builders to 
set carburettors so 

j. 3 o to § ive maximum 
power, instead of 
aiming at mini- 
mum production of CO and therefore maximum thermal 
efficiency. 

Mr. Dugald Clerk has also carried out tests of this kind. 
He used for this purpose the engine on his 18 h.p. Siddeley 
car. The engine was a 4-cylinder one, bore 4 inches, stroke 
4 inches. Samples of the exhaust were taken while — 



^ % %co 2 
5*, /o0 2 


10-9 12-813-5 10-6 96 60 


3-6 1-5 


f § %?o 


07 50 625 11-6 


^•S %" 2 


2-1 6-65 8-7 


«M 


30 


I 11 




















28 




A 




- § 8 26 












\ 
















0-26 


A 












s 024 
a 

£ 5 0-22 


T 




B 








— 


\ 


\ 








"5"S o-2o 




\ 


\ 






Therm 
-eckon 

CD 








N 


\ 




OlBi 










N 


\ 



018 



0-20 0-22 0-24 026 
Petrol Consumption 

Fig. 112. 



0-2& 



+3 
















fl 




o r . S S 


en 


CM 00 CO 


CP 00 CO CO 

^ i—l oo 


o 




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PH 
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w 


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3 ^ H £ 


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C3 pit CM CO 


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w. 
w 


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CO • 


ft 




£ «> 


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o 




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h66 


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JS vo O i-H 


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CD O 


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§ 00 (M Ol 


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43 -- 1 go 


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« CO h (M 


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309 




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ri 
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C 

c 



310 THE INTERNAL COMBUSTION ENGINE 

(a) The car was standing on the level with the engine 

running as slowly as possible. 

(b) The car still standing, but engine running at about 

600 r.p.m. 

(c) The car running on a level at about 18 m.p.h., the 

throttle less than half open. 

(d) The car climbing a hill, engine running about 1,000 

r.p.m., and throttle from three-quarters to full 
open. 

It will be seen from the above table that under circum- 
stances which might quite often occur in practice about 4 
per cent, of CO is being produced. To meet the difficulties 
of designing a carburettor which should mix air and petrol 
in constant proportions under all conditions of load and speed 
is no easy thing, indeed most builders aim at quite different 
mixtures viz., those that make for ease at starting, for 
rapidity of "picking up," and other features of car manage- 
ment that make for ease of manipulation. Mr. Dugald 
Clerk is so sensible of its difficulty that he has not hesi- 
tated to make the suggestion contained in the following 
extract from his paper — 

' ' As the problem is to charge any given volume of air passing 
into the engine with a practically unvarying proportion of 
petrol vapour in an uniform manner, it seems to me that all 
systems of speed control must fail to obtain proportionality 
throughout the whole range. If the entering air could be 
made to drive a fan, similar say to an anemometer, with 
proper precautions this fan or anemometer could be made to 
run at a speed of rotation very closely proportional to the 
volume of air supply throughout the whole range of con- 
ditions. If a spindle be driven by this anemometer, and a 
small chain passed from that spindle into a petrol vessel at 
constant level, then it would be possible to work out a carbur- 
etting contrivance which would supply the air passed through 
it on its way to the engine with petrol exactly in proportion 
to the charge volume taken in. I believe contrivances of 
this kind have been proposed, and it seems to me that 
although we have practical difficulties, if correctly carried 
out, they should be able to give a perfectly uniform mixture 



PETROL ENGINE EFFICIENCY AND RATING 311 

to an engine under all conditions of speed variation and 
charge volume variation." 

Mr. Dugald Clerk concludes from the results of his experi- 
ments that the following conditions appear to produce 
imperfect combustion — 

1. " Too rich mixture with insufficiency of oxygen. 

2. Too weak mixture with excess of oxygen, but too slow 

a rate of ignition and combustion. 

3. Irregular mixture — mixture supplied too rich in com- 

position at one part of the stroke, and too weak in 
another ; that is, bad mixture. 

4. Engine and carburettor cold. This tends to cause im- 

perfect combustion, due partly to low temperature 
and partly to bad carburetting. 

5. Improper timing of ignition, and missed ignitions. 

6. Igniting in the body of the cylinder, instead of in a port. 

This will produce imperfect combustion at light 
loads." 

The sixth of the above conditions is an exceptionally 
interesting one. It seems incontestably to be the case 
that the presence of " pockets " in cylinders leads to loss of 
efficiency, "pockets "being the name given to any recess in 
the top of a cylinder which has the effect of increasing the 
clearance volume. But it is almost equally certain that the 
presence of "pockets" improves the running of the engine 
under working conditions : and renders it, as it is termed, 
" more flexible." Pockets lower the efficiency as they 
increase the ratio of surface to volume, but they render igni- 
tion more certain, as, even at light loads when the exhaust 
products left in the clearance space dilute the incoming 
charge very considerably, there is the likelihood of the neigh- 
bourhood of the sparking plug (which is probably situated in 
or near one of these pockets) being rich locally in explosive 
mixture, so ensuring the proper starting and timing of the 
ignition. For this reason pockets are popular. 

146. Combustion of Paraffin. — Average paraffin consists 
chiefly of decane of which the chemical symbol is C l0 H -. 
The combustion of decane in air may be represented 
chemically by the formula 



312 THE INTERNAL COMBUSTION ENGINE 

2C 10 H 22 +3lO 2 + 116-5N 2 =20CO 2 +22H 2 O + 116-5N 2 

The composition by volume of the exhaust gases when 

cooled to normal temperature and pressure will therefore be 

20 

or 14*6 per cent, of C0 2 and 85*4 per cent, of nitrogen. 

136-5 * * & 

If 10 per cent, less air were admitted the combustion would 
be incomplete, as on the average 6*2 atoms of carbon would 
get oxidized to the extent of CO only. This would lead to 

62 

there being - or 5 per cent, by volume of CO 

& 104-9 + 6-2 + 13-8 r J 

present in the exhaust gases. These exhaust gases are not 
identically the same as those produced in the working of an 
internal combustion engine which uses paraffin as its fuel for 
two reasons — 

(a) Paraffin is not pure decane. 

(b) The gaseous mixture which is ignited in the cylinder 
consists in part of some of the exhaust products of the previ- 
ous explosions and the composition must depend a good 
deal on the history of the explosions of the last few strokes. 
The effect of governing by throttling, and still more by " hit- 
and-miss," is to affect the composition even more strongly. 

From the equation given above the ratio by volume of air 



to paraffin vapour for complete combustion is — — 



or 



74 to 1 as against about 45 to 1 for average petrol. On the 

other hand, paraffin weighs a good deal more per unit volume 
than petrol and on the whole there is little difference in output 
for a given cylinder capacity between the two. From tests 
that have been carried out it is pretty clear that the 
number of miles run per gallon of fuel on the same car is 
not very different whether petrol or paraffin is used. 

147. Road and Air Resistance. — The performance of a 
petrol or paraffin engine as measured by the number of 
gross-ton-miles run by the vehicle per gallon of fuel depends 
upon the resistances encountered by the car in its motion. 
The total resisting force can be divided up into three parts, 
one independent of the speed, the second proportional to 



PETROL ENGINE EFFICIENCY AND RATING 313 

the speed, and the third proportional to the square of the 
speed. Symbolically — 

E = a+bv+Cv 2 
where B = total resistance in pounds 

v = speed of car in miles per hour 
and a, b and c are constants. 

The constant a is chiefly related to the road resistance, 
the term b to the Motional resistances of the mechanism, 
and the term c to the air resistance and probably also to 
the throttling of the gases through the ports when the 
engine is racing. Experiments to show with accuracy 
what the values of the constants a, b and c .should be, are lack- 
ing. With bicycles a good deal of work has been done, and 
Professor Perry many years ago gave the following formula 
for the total resistance to motion — 

80V 20 200/ 
where R is measured in lb. ; IT is the total moving weight 
in lb. and v is speed in m.p.h. The author, as the result of 
some experiments carried out at Cambridge,* found 

B=™-(l+JL+*-\ 
132V 20 84/ 

The method by which this last-named formula was 
obtained was by noting the time taken to come to rest from 
a definite velocity when travelling over a flat road.f The 
same method could be used even more simply on a motor 
car by simply cutting off the spark and watching the speed 
indicator. No such experiments appear, however, to have 
been made. 

148. Mr. Edge, however, has carried out some interesting 
tests at Brooklands to see what effect the raising of a large 
wind screen would have on the speed of a 6-cylinder 
Napier car giving 38-4 h.p. on the R.A.C. rating. His 
results were — 



* Mechanical Engineer, November 11, 1899; UEctairagc Elec- 
trique, November 30, 1901. 

t In this case 2*=— tan- 1 - i!L wh(M1 / = \ i^ZjjT 
fg bv + 2a 



314 THE INTERNAL COMBUSTION ENGINE 



Area of Wind Resistance Screen 
in Square Feet. 


Speed in M.P.H. 


30 


47-85 


28 


500 


26 


52-9 


24 


5615 


22 


540 


20 


55-5 


18 


57-0 


16 


57-6 


14 


60-0 


12 


62-5 


10 


64-2 


8 


6615 


6 


70-25 


4 


750 


2 


73-8 





790 



These figures are shown plotted in Fig. 113. If it be 
assumed that the engine was driven during each run so as 
to give its maximum power it is interesting to work out 
the following exercise. 

149. Exercise. — Let the cv 2 term in the resistance equa- 
tion be written as f(A -\-s)v 2 where A is the area of the wind 
screen, s is the effective area of the front view of the car 
and / is a constant. 

Then R = a+bv+f{A+s)v 2 . 

Now the horse-power of output will be proportional to E xv, 
and if it be assumed that the engine is always trying to 
exert approximately the same horse-power then 

v x {a+bv+f(A+s)v 2 } 
will be a constant quantity. 

Now the value of this when A =0 is 
79{a+796+(79) 2 / < sj 

To avoid too much arithmetic at this stage write v in 
place of 79 and it follows that at all speeds the following 
relation must hold 

v{a + bv + f(A+s)v 2 )=v {a+bv +fsv 2 } 

v G (a+bv +fsv 2 ) 



or 



v*+—~ — v 2 + 
a cubic equation in v. 



-v- 



f(A+s) 



PETROL ENGINE EFFICIENCY AND RATING 315 

As this equation only contains two variables v and A, 
it shows directly how v will vary for different values of 
A. If the constants a, b, / and s were known it would 
be possible to calculate the value of v for, say, A =30 as in 



45 



40 



25 













































\ 




































N 


\ 


a 


































\ 




o 


































\ 






































\ 




































\ 






































\ 


\ 










































































<J\ 


\ 



















































































































• 








































V 
























oQt 


serv 


ed I 


'alue 


s 























































































































40 



80 



45 50 55 fffl G5 70 75 

.Syo eeaf in Miles per Hour. 

Fig. 113. — Diagram showing the effect of wind resistance on speed, as 
found by Mr. Edge on a Napier Car carrying a special wind screen. 



85 



the first line of the preceding table. If the constants were 
correct v would come out at 47-85 m.p.h. as found by 
Mr. Edge. An interesting exercise that now arises for 
students to undertake is this. Reverse the above process. 
Given the curve (as in Fig. 113), find the values of the 
constants a, ?>, / and s. As regards s it may be pvotty safely 



316 THE INTERNAL COMBUSTION ENGINE 

assumed as about 12 square feet to start with. The author 
has not himself solved this exercise, nor, so far as he knows, 
has any one else. It is therefore possible for an enter- 
prising student to do this and really find out for himself 
what are the constants in the equation 

B= a-\-bv-\-cv 2 
as applied to this car. 

A good many suggestions have been made as to certain 
of the values of these constants taken alone. Thus the 




o to 20 30 40 50 60 7o SO 90 100 110 120 
Miles per hour 

Fig. 114. — Showing relation between speed and H.P. necessary to over- 
come wind resistance per 12 sq. ft. of area. 



road resistance is known to be not usually more than 50 lb. 
per ton, although cases of 100 lb. per ton are known. Also 
there are estimates of air resistance, and in Fig. 114 is 



PETROL ENGINE EFFICIENCY AND RATING 317 

reproduced a curve based upon data given by Mr. Strickland 
in his valuable book on motor cars. He states that it is 
founded on information supplied by Col. Crompton. 

150. Exercise.— liv{a+bv+f(A+s)v 2 } = v (a+bv +fsv 2 ) 

find the value off — | i.e. the rate at which the speed is 

V dAJ ■ 

reduced as the area of the wind screen is increased. 

Differentiate the above equation 

^ {a+hv+ f(A + ^}+v {b ^ + 2f { A +s)v ^ 



or —{a+bv+f(A+s)v 2 +bv+2f(A+sy-}+fv«=0 

CtJx 

dv \ fv 3 



giving v dAJ 0+2 &v+3/(4+»)v a 

When the student has worked out the values of a, b and 
/ he can test the accuracy of this method of finding the 
slope of the curve in Fig. 113. Or he may, if he likes, pro- 
ceed the other way round. 

151. Efficiency of Motor Vehicles.— A standard of com- 
parison early adopted was the measurement of miles run 
per gallon of fuel. This figure used to come out at about 
40 for small touring cars and about 5 or 6 for the heavy 
vehicles used in commercial work. The great range of 
values of this standard made it difficult to compare cars 
with each other, and it is not difficult to see why this is so. 
The number of miles run per gallon is merely a measure of 
the distance through which the car is moved for a given 
number of thermal units supplied. To make the comparison 
real it is necessary to introduce some factor which takes 
account of the resistance overcome during the motion of 
the car. This resistance is proportional to a term of the 
form 

(a-{-bv-\-cv 2 ) 
where v is the speed of the vehicle. 

For slow-moving cars this expression is practically equal 
to a and is a constant proportional to the total moving 



318 THE INTERNAL COMBUSTION ENGINE 

weight. In other words the resistance overcome is pro- 
portional to the total moving weight. To express, therefore, 
the work done in moving the car over a given distance it 
is sufficient to multiply the distance travelled by the weight. 
In this way we arrive at the " gross-ton-miles-per-gallon " 
as a satisfactory standard of comparison. In the instances 
above given the gross-ton-miles-per-gallon would be about 
30 for the small touring car and about 35 for the heavy one. 
That the touring car gives a lower result is due in part to 
the fact that in the case of rapid speeds (say over 20 miles 
per hour) air resistance as well as road resistance has to be 
allowed for, and that economy in fuel is more sought after 
in a commercial vehicle than in a pleasure one. 

Very interesting series of trials of touring and commercial 
vehicles have been organized by the Royal Automobile Club, 
and from the figures given in their bulky reports the following 
tables have been made out — 

1907 Trials of Commercial Vehicles. 



Average provided 
Xet Load carried by Vehicles. H.P. (R.A.C.) per ton of 
| Gross Moving Load. 



| ton (5 cars). 

1 ton (4 cars) . 
1^ tons (5 cars) 

2 tons (7 cars) 

3 tons (15 cars) 
5 tons (1 car). 




Average G.T.M. per 
Gallon of Petrol 



27-4 

33-26 

30-9 

3493 

40-6 

33-9 



R.A.C. International Touring Car Trials, 1908. 



Miles run per Average weight 
H.P. (R.A.C. rating) gallon (average of (loaded) of best 
best cars). cars. Tons. 



Up to 20 h.p. 

20 to 40 h.p. 
40 to 60 h.p. 




Gross-Ton-Miles 

per gallon of petrol. 

Average for best 

cars. 



1-30 
1-72 
2-14 



30-7 
29-6 
33-9 



Xote. — Total distance run in the 1908 trials was 1,977 miles on the 
roads in Scotland and England. Average speed of all cars probably 
between 15 and 20 miles per hour. 



PETROL ENGINE EFFICIENCY AND RATING 319 

These results, taken into consideration with others the 
author has had to deal with, suggest the figure of 35 as a 
good one to use as a convenient standard of performances 
of commercial vehicles when running on petrol — on 
paraffin, 30 is a fair figure to use. For touring cars it is less 
easy to quote a standard as the speeds have to be taken 
into account. One commercial car in the above trials (a 
Maudslay car fitted with the White and Poppe Carburettor, 
described in the previous chapter) achieved the phenomenal 
figure of 62, and it is of interest to see what such a good 
result means. It happens that the " G.T.M. per gallon " is 
really a measure of thermal efficiency. Thus if the road 
resistance be put at 50 lb. per ton and the number of foot- 
pounds stored up in each pound of petrol be 15,000,000, 
then the above performance shows a thermal efficiency of 
the whole car of 

50 x62 x5,280 , 

= about 15 per cent. 

7-2x15,000,000 

This means that of every 100 units of energy in the fuel 
fed into the engine no less than 15 units were transmitted 
to and used by the road wheels. Putting the loss between 
the crankshaft and the road wheels at 30 per cent, this 

corresponds to a ratio of — — of 21 per cent., 

thermal energy put in 

which is an exceedingly good figure seeing that it repre- 
sents a general average of running at low and high speeds 
and all conditions of power output. With touring cars 
the figure of comparison is, as has been stated, in the neigh- 
bourhood of 26 (or 20 when running on paraffin) for a car 
of which the top speed is 20 miles per hour, going down to 
20 as the maximum speed rises continuously to 40 miles 
per hour. Owing to the intervention of the air resistance 
at high speeds, the G.T.M. per gallon is not truly pro- 
portional to the thermal efficiency of the car. The thermal 
efficiency is really the G.T.M. per gallon multiplied by 
c^+fa+cv* ^ / l+ b v _^c_ v2 \ The ratiQ Qf aetual horge 

a \ a a I 

power used to total weight moved should, of course, be 



320 THE INTERNAL COMBUSTION ENGINE 

proportional to the velocity and could be used in calculating 
it, but in the first of the tables given on p. 318 the stated 
horse-power is only that which the engine could on certain 
suppositions exert as a maximum. It will be noted that 
the ratio falls as the load rises, so giving a general 
resemblance to the way in which the speed also falls as the 
vehicle gets heavier. 

PROBLEMS. 

1. Describe, with sketches, an indicator which has been 
used to indicate petrol engines at 2,000 revolutions per 
minute. What were the results of Professor Callendar's 
investigations ? (B. of E., 1907.) 

2. If a car is travelling along a level road find the time 
taken for the speed to fall when the ignition is switched 
off, from v to v x and the space covered during this time. 



INDEX 



References are to "pages 



Absolute zero of temperature, 12 
Acetylene, 217 
Acme gas engine, 183 
Actions, molecular, 3 

tidal, 3 
Adam, 187 
Adams engine, 247 
Adiabatic expansion, 16, 17. 19, 

70, 80 
After-burning, theory, 38 
Air, heat contents of, 32 

standard, 31, 89, 90, 96, 296 
Albion engine, 242, 244, 247, 

254, 257, 281 
Alcohol, 223, 224, 227 
Aluminium, 216 
Ammoniacal liquor, 225 
Ammonium sulphate, 190, 212 
Andrews, 192 
Anna colliery, 211 
Anthracite, 173, 186 

oils, 225 
Austin, 2, 90, 97 
Ayrton & Perry, 66 



Bailly & Kraft, 209 

Bairstow, 51, 95 

Balancing, 164, 165, 166, 167, 

168, 169 
Balantyne, 187, 309 
Beardmore & Co., 116, 199 
Benzol, 223, 229 

" 90 per cent,," 225, 229 

" 50 per cent.," 225 
Benzole, 212, 225 
Besant (Sir Walter), 5 
Bessemer works, 216, 220 
Bibbins, 191 
Bituminous coal, 188, 189 



Blast-furnace gases, Chap. VII 

Blount, 187, 217 

Bonnet, 247 

Borneo oil, 223, 271 

Bosch Simms magnets, 279, 285- 

289 
Boyle's law, 12 
Brake horse-power, 143 
Brayton engine, 29 
Broom & Wade paraffin car- 
burettor, 264 
Burrows, 189 

Burstall, 39, 50, 71, 78, 132 
By-product coal tar, 225 

coke-ovens, 212 

recovering, 190 



Calcium carbide, 216 

cyanamide, 218 
Callendar, Prof., 40, 78, 295 
Calorie, 11 

Calorific value alcohol, 224 
coal, 36 

gas, 36, Chap. VI 
of paraffin, 224, 261 
petrol, 36, 224, 261 
petroleum, 36 
Calorimeter, Junker's, 155 
Campbell engine (gas), 131, 140. 
141 
(oil), 231, 232, 233, 234 
producer, 179, 180, 181. 185, 
200 
Carbolic oils, 225 
Carbon-dioxido. calorific value oi, 
175 
monoxide, calorific value of, 
175 
in exhaust. 307. 309 



321 



322 



INDEX 



Carburettors, jet, theory of, 266 

for paraffin, 258, 264, 265 

for petrol, 226, 243, 249, 250, 
251, 252, 253, 257, 262 
Cargo Fleet Iron Works, 123 
Carnegie Steel Co., 220 
Carnot cycle, 20, 26 
Charles Law, 12 
Chatelier, Le, & Mallard, 2, 38, 

53, 67, 68, 97 
Chemical manure, 217 
Cleaning gas, 213 
Clerk, Dugald, 1, 2, 37, 39, 52, 
81, 130, 134, 149, 270, 
308, 311 
Clutch friction, 246, 247 
Coal, anthracite, 173, 186 

bituminous, 188, 189, 197 

Distillation Co., 212 

tar products, 225 
Coal-tar, 189, 212 
Cockerill Company, 210, 220 

engine, 123, 125, 127, 129 
Coil ignition, 274 

induction, 272 
Coke, 186, 217 
Coke oven gases, Chap. VII, 210 

beehive type, 212 

Hiiessener bv-product, 212 
Coker, Prof., 105, 106 
Combustion and explosion, Chap. 
Ill 

of paraffin, 311 
Composition of gases, 25, 213 
Compression pressure, alcoholic 
engines, 228 
gas engines, 130, 208-12 
petrol engines, 228, 261 

ratio, 11, 28, 133, 134, 302 
Commutator, 274, 275 
Connecting rod motion, 165, 166, 

167, 168, 169 
Constants, table of, xi, xii 
Coster, 195 
Costs, 191 
Cottrell paraffin carburettor, 258, 

259, 260, 261, 262 
Cooling, 38 

Crompton, Colonel, 317 
Crosby indicator, 136, 138, 139 
Crossley engine, 130, 132, 155,195 
Crude naphtha, 225 



Cycles, Chap. II 
Carnot, 20, 26 
Clerk, 10, 115, 126 
constant pressure, 10, 27 
temperature, 10, 26 
volume, 10, 29, 81 
Otto, 10, 115 
Cyclic irregularity, 158, 160, 161, 

162, 163, 164 
Cylinder dimensions, effect of, 
295 



Daimler engine, 291, 307 

Dalby, 78, ^164, 186 

Davenport, 285 

Decane, 311 

De Dion engine, 294 

Diagram, entropy, 19, 23 

Diesel engine, 11, 29, 31, 231, 

237", 238, 239 
Differential, 246, 248 
Dissociation, 38, 175 
Donkin, Bryan, 209 
Douglas, 46 
Dowson, 177, 179, 189 
Duckham, 230 
Dudbridge gas engine, 185 

Edgar Thompson Works, 220 
Edge, S. F., 313, 314, 315 
Efficiency " Air Standard," 31, 
89, 90, 96, 296, 298 

effect of cylinder dimensions, 
295 

improving in gas engines, 130 

of motor vehicles, 317 

new expression for, 85, 86, 92, 
111 

with var. sp. heats, 81, 85, 
87, 90, 96 
Ehrhardt & Sehmer engine, 128 
Eisemann electric ignition, 272 
Electrical degrees, 160 
Electric ignition, 271, 289 
Energy, intrinsic, 62 
Engines, gas, Chap. V 

oil, and petrol, Chap. VIII 
Engine, petrol rating, 295, 299 
Entropy, 62 

diagram, 19, 23 
Eschweiler Mining Co., 211 



INDEX 



323 



Exhaust gases, composition of, 

305 
Expansion, adiabatic, 16, 17, 19 

isothermal, 17, 19 
Explosion and combustion, Chap 
III 
Bairstow & Alexander, 51 
experiments, Clerk, 37 
Douglas, 46 
Grover, 42 
Hopkinson, 55 
Massachusetts Institute, 50 



Fielding engine, 133, 135 
Flashpoint, 226 

Flexibility of petrol engines, 244 
Float chamber, 250, 259, 260 
Flow of heat, 99, 100, 105 
Flywheel effect, 157, 158 
Forrest, James, lecture, 134 
Friction-Clutch, 246, 247 
Fuels Committee of Motor Union, 
227 
for oil and petrol engines, 223 

Gahrungsversuchsanstallt, 228 
Gardner engine, 185 
Gas, cleaning, 198, 213, 214, 215 
composition of, 25, 177, 178, 

179, 188, 189, 191, 198, 

210, 213 
engines, Chap. V 
Acme, 183 
Campbell, 131, 140, 141, 183, 

185 
Cockerill, 123, 125, 127, 129 
Crosslev, 130, 155, 183, 185, 

195 
Dudbridge, 185 
Ehrhardt & Sehmer, 128 
Fielding, 133, 135, 185 
Gardner, 185 
governing of, 156 
Hindley, 185 
Koerting, 116, 118, 119, 120, 

121, 122, 137 
Kynoch, 185 
National, 116, 117, 130, 132, 

183, 185 
Newton, 185 
Oechelhauser, 116, 123, 124 



Gas engines, Paxman, 185 
Premier, 130 
Railway & General, 185 
Tangye, 183 
Thornycroft, 197 
Westinghouse, 130, 220 
perfect, 12, 63 
producer, Chaj). VI 
Gases, exhaust composition of. 

305 
Gasolene, 224 
Gear box, 246 

German Association of Engin- 
eers, 153 
Governing of gas engines, 156 

petrol engines, 254, 255, 256 
Greiner, 211, 219, 220 
Gross ton miles per gallon, 

measurement, 318, 319 
Grover, 42 



Haber, 97 

Harmonic motion, simple, 165, 

167 
Harrison, J., 168 
Heat, 3 
Heat balance-sheets, 148-153 

contents of air, 32 

energy (unit of), 11 

flow of, 99, 100, 105 

Heintz, 128 

Hexane, 224, 306 

specific, 13, 38, 92-98 

unit, 11 

volumetric, 94 
High-tension coil, ignition, 274 

magneto, ignition, 277 
Hindley gas engine, 185 
" Hit-and-miss " governor, 156, 

312 
Hoffmann, 209, 211 
Holborn, 2, 90, 93, 97 
Hopkinson, Prof., 2, 51. 55. 58. 
90, 95. 141, 142. 224, 291, 
293, 307 
Hornsby oil engine, •235. 271 
Horse-power, brake. 143 

indicated, 136 
Hubert, 200. 210 
Hiiessener by-product coke 
ovens, 212 



324 



INDEX 



Hydrogen, calorific value, 174, 
190 
contents in gases, 191 

Ignition for gas, oil or petrol 
engines, 233, 236, 270-289 
electric, 271-289 

high-tension coil, 274, 276 
high-tension magneto, 279 
low-tension magneto, 277 
spark, character of, 282, 283, 

284, 285, 294 
timing of, 280, 281 
tube, 270, 271 
Indicator, analysis of motion, 
144-148 
Crosby, 136, 138, 139 
reflecting, 140, 290, 291 
Induction coil, 272, 273 
Institution of Civil Engineers, 24, 
32, 134, 141, 193, 297 
of Naval Architects, 194 
Insulation resistance of spark 

plug, 294 
Intrinsic energy, 62 
Irregularity, cyclic, 158, 160, 

161, 162, 163, 164 
Isothermal exnansion. 17, 19 



Jet carburettors, theory of, 266- 
269 
of carburettor, 250, 252, 253. 
257, 263, 266 
Johannesburg plant, 121 
Joule, 11, 16^ 

equivalent, 11 
Junker's calorimeter, 155 



Kerosene, 224 

Koerting engine, 116, 118, 119, 

120, 121, 122, 137, 151, 

212 

Koppers, 211 

Krebs carburettor, 252, 269 

Kynoch gas engine, 185 



Lag of ignition gear, 281 
Lanchester engine, 245, 251 
Langen, 50, 90, 97 



Law, Boyle's, 12 

Charles, 12 

of thermodynamics, 62, 63 
Leyden jars, 276 
Lewes, Vivian B., 224 
Lime, 217 
Lodge ignition, 276 
Lodge, Sir Oliver, 4, 276 

" Make and break " of coil, 274 

Mallard & Le Chatelier, 2, 38, 
53, 67, 68, 97 

Manure, chemical, 217 

Marine propulsion, 193 

Mather & Piatt, 116 

Mathot, 128, 158 

Maudslay engine, 257, 319 

Maxwell cars, 226, 227 

McKechnie, 194 

Mesati ores, 216 

Miles per gallon measurement, 
258, 312, 317 

Milnes-Daimler carburettor, 262 

Milton, 193 

Mirrlees Watson & Co., 233 

Molecular action, 3 

Mond producers, 178, 190 

Morse, 307 

Motor canoes, 242 

car, arrangements of, 246 
vehicles, efficiency of, 317 

Murray governor, 254 

magneto ignition gear, 281 

Naphtha, crude, 225 

solvent, 225 
Napier engine, 315 
National gas engine, 116 
Needle valve, 250 
Newton gas engine, 185 
Nicolson, 155 

Oechelhauser engine, 116, 119 
O'Gorman, 225 
Oil, Borneo, 223, 271 
coal tar products, 225 
engines, Chap. VIII 

Campbell, 231, 232, 233, 234 
Diesel, 11, 29, 31, 231, 237, 

238, 239 
Hornsby, 235, 271 



INDEX 



325 



Oil, paraffin, 223, 258, 264 
Operation of producer plant, 200 
Ormandy, 228 
Otto cycle, 10, 115 



Paraffin carburettors, 258, 264, 
265 
combustion of, 311, 312 
Parallel, driving generators in, 

130, 158, 160 
Paxman engine, 185 
Pennsylvanian petroleum, 224 
Perfect gas, 12. 63 
Perrv, Prof., 64, 67, 164, 266, 

291, 313 
Petavel, 67, 76 
Petrol, 223, 305, 306 

consumption, 292, 294 ; see 
also miles per gallon and 
gross ton miles per gallon 
engine efficiency, Chap. IX 

rating, Chap. IX 
engines, Chap. VII 
Adams, 247 
Albion, 242, 244, 247, 254, 

257, 281 
Daimler, 291, 307 
De Dion, 294 
Lanchester, 245, 251 
Maxwell, 226, 227 
Maudslay, 257, 258, 319 
Napier, 315 
Siddeley, 308 

Thornycroft, 239, 240, 241 
Piston, gas engine, 132 
oil engine, 234 
petrol engine, 243 
" Pockets," 304, 305 
Power, brake horse, 143 
indicated horse, 136 
utilization of surplus, 216 
Pratt's motor spirit, 224, 307 
Pre-ignition, 130, 191, 208, 212 
Producer gas, theory of, 173-179 
Campbell, 179, 180, 181, 185, 

200 
Mond. 178 

National, 180, 182, 185 
Poetter, 121 

suction, operation of, 200-207 
Propeller shaft, 246 



R.A.C. motor vehicle trials, 258, 
318 

rating, 298, 299 
Radiation, 76 
Radio-activity, 3 
Railway and General gas engine, 

185 
Rating of petrol engines, 295 r 

299 
R.A.S. trials, 181 
Ratio of compression, 11, 28 

of specific heats, 15, 86 
Regnault, 97 
Resistance, air, 312, 314, 315 

road, 312 
Richardson, Westgarth & Co., 

125, 214, 215 
Road resistance, 312 
Rossi, 209 
Rotter, Max, 209 

Saarbruck, 128 

Sankey, Captain, 24, 186 

Schiller, 160, 164 

Scottish Producer Trials, 182,. 

183 
Scrubber, 180, 181, 182, 186, 

187, 202, 205, 212 
Sehmer, Ehrhardt and, engine r 

128 
Shelton Iron Works, 212 
Ship Propulsion, 193 
Siddeley Engine, 308 
Siemens generators, 121 
Simple harmonic motion, 165, 

167 
Simms-Bosch high-tension mag- 
nets, 279, 285-9 
Solar heat, 3 
Southern Nigeria motor canoes , 

242 
Spark, ignition, nature of, 282, 

283, 284, 285. 294 
Sparking plugs, 274-8 

insulation resistance of. 294 
Specific heat, at constant pres- 
sure, 13 
at constant volume, 13, 95, 96 
Increase o\\ 38, 67. 81, 92 98, 

111 
measurements, 55, 92, 95, 1 1 1 
Ratio ot 15, 86 

/. 



326 



INDEX 



Standard air, 31, 89, 90, 96, 296 
Statistics, consumption of par- 
affin and petrol, 262 
distribution of power in motor 

cars, 249 
petrol supply, 230 
petroleum supply, 225 
power, 6, 208 
Strickland, 276, 317 
Suction producers, 174 
Suction types, see producers 
Surplus' power, utilization of, 

216 
Symbols, chief, tables of, Ch. 
XIII 



Table of constants, xi, xii 
Tangye engine, 183 
Tar-coal, products, 225 
Temperature, absolute zero of, 
12 
measurements, 78 
variation in cvlinder, 58, 75, 
100, 103, '105 
Tests, gas engine 142, 153, 154, 
156 
oil, and petrol engines, 237, 238, 

291, 292 
producer, 155, 181. 183, 185, 
186, 188 ' 
Theisen gas washer, 213, 214, 215 
Theorv of jet carburettors, 266- 

"269 
Thermodynamics, Ch. IV 
Thermodynamic cycles, Ch. II 
Thompson, Professor S. P., 217 
Thompson, 209 
Thorny croft gas engine, 197 
paraffin carburettor, 265 
petrol engine, 239, 240, 241 
Thwaite, 209, 219 
Tidal action. 3 



Timing of ignition, 280, 281 

Tisdall, 294 

Topham, 294 

Trembler of coil, 274, 275, 277 

Tube ignition, 270, 271 



Unit of energv, 11 

of heat, 11 ' 
United States Steel Corporation, 

220 
Utilization of surplus power, 216 



Valves, gas engine, 116, 117. 123, 

125 

needle, 250 

oil engine, 233, 234, 238, 239 

petrol engine, 243, 246 
Vaporization, 228, 265 
Vaporizer. Campbell, 233 

Hornsby, 235 
Vehicles, motor efficiency of, 317 
Vena contracta, 256, 257 
Vickers, Son & Maxim, 194 
Volumetric heat, 94 



Waal, Van der, 98 

Washer. Theisen gas, 213, 214, 

215 
Waste power, from blast fur- 
naces, Chap. VTI 
from coke ovens, Chap. VTI 
Wath Main Colliery, 212 
Watson, Prof., 282, 283, 284. 285 
West ingho use gas engine. 220 
White & Poppe carburettor. 257. 
319 



Zero, absolute (of temperature), 
12 



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